You should know a couple of things before reading folks:
(1) This is intended as a supplementary guide only, and it will have a higher chance of working for you if you read the entire thing (it's not designed for tech-savvy people, this is to help those who look at phone specifications and go 'What the...?').
(2) If you are still unsure after reading this, I encourage you to do some extra research. There are some excellent sites out there, and some fantastic reviews. You can also send me a message and ask (whether I'm selling the phone or not!) If I can help, I will!
Right! As someone who has owned many mobile devices over the years, I have decided to pass on what knowledge I have in the hope that I may be able to spare you the trouble and frustration of purchasing the wrong Mobile Phone on eBay. There's nothing worse than buying a brand new phone, only to receive it and find it to be incompatible with your network. Those of us with Optus and VHA are largely okay, as the vast majority of devices sold on eBay today are completely compatible with these networks. For those of us with Telstra, or on the new Vodafone 850 network, extra care must be taken when selecting a handset.
The reason behind it is very simple once you understand. Optus, Vodafone* (now VHA after merging with Hutchison '3'), Virgin etc. all use either 900 or 2100MHz for their 3G (third generation) cellular networks (you will see 3G commonly referred to as WCDMA, UMTS and HSPA in seller listings - they all mean the same thing). All handsets provided by these companies and their subsidiaries (Crazy John's, Boost, Amaysim, Westnet, Woolworths Everyday etc.) support these frequencies. Additionally, 3G networks in Asia and Europe as well as T-Mobile in the UK and the US (in some parts at least) also use these frequencies. That is why the vast majority of phones which can be purchased from Hong Kong and Chinese sellers, as well as from Europe, largely only support these bands.
* This guide has been updated to reflect Vodafone's 850 network. If purchasing for this network, you need to follow the same instructions as for NextG!
In the interests of improving overall coverage, Telstra chose to use the 850 band for their NextG network (it travels further per tower and penetrates buildings more effectively). This means that handsets for NextG/Voda850 must be equipped with an 850MHz 3G radio. Traditionally, 850MHz handsets are most common in the US and Canada, thanks to AT&T and Rogers/Telus respectively. Telstra/Vodafone do a reasonable job in providing a good range of handsets, but at the end of the day they are sold at retail prices, which is something we (or certainly I) try and avoid here on eBay!
It's probably worth noting here that 2G networks - also called GSM networks - are less of an issue. Most modern phones are 'Quadband GSM' meaning they support all frequencies worldwide. This is great for voice calls and SMS, but not so good for data where they crawl along at a snail's pace. An Asian or European handset will work just fine on Telstra's GSM network, but you don't get the extended coverage and data speed of the NextG network. Also note that if your phone lists 850MHz under GSM frequencies, that doesn't mean it'll support NextG/Voda850.
A quick note about newer 4G (LTE) networks: In Australia, LTE networks run on 1800MHz currently. That means your phone will not only need an 850MHz 3G radio, it'll also need an 1800MHz 4G LTE radio if you want the best. Be aware there's a lot of marketing malarkey around '4G', and that American carriers actually call our NextG '4G'. Also of note, phones designed to run on AT&T, Rogers, T-Mobile, Verizon etc. LTE networks, won't get LTE in Australia. If you need LTE currently, look towards Korean handsets (some Korean LTE networks use 1800MHz), or buy local.
Now, boring stuff out of the way, bring on the good bits!
Buying an 850MHz handset can be an exciting experience, but it can also involve frustration (and lots of postage) if you aren't 100% sure what you need. The easiest way is just to make sure the handset came from Telstra, or is a newer handset from Vodafone. The listing may say 'Telstra stock' 'Unlocked from Telstra' 'Unwanted plan upgrade from Telstra' or something similar. If it's a newer phone, it'll work with NextG. If it's an older model you're looking at, make sure first. Don't be scared to ask questions. No question is foolish, and at the end of the day you're the one paying the money! A good seller will always include in the listing whether a phone is NextG compatible or not (thanks largely to the fact that 850 handsets - especially in the top-tier devices - can command a price premium). Something to be wary of is that not all sellers know as much about these things as you do (after reading this at least!) so you may have to do a little research.
There are ways to work it out for yourself if the information is provided. They're largely manufacturer unique and rely on model numbers etc. so I'll cover the most common ones here to make life easy!
Quick Side Note: Something else to be wary of are the 'advanced specifications' details eBay automatically lists for phones (when you do the 'model number' search when creating the listing). They are not always correct (especially with 850 handsets, they seem to default to European bands/models), and just because they say 900/2100 MHz does not always mean that particular listing is. This can work in your favour as a buyer, as some people simply look at these tables and see 900/2100MHz 3G, then cease watching that product. Simply asking the seller can often yield results (and sometimes an excellent price - take it from someone who's been lucky enough to benefit a few times). Naturally if you are selling this may work against you (reducing number of 'interested' visitors and putting off genuine bidders). Always double-check them first, and if your handset differs don't include them. You can always add your own specifications to your listing and leave out all the junk!
Here's how to tell some common manufacturers' phones apart:
(1) HTC. The key to understanding which networks HTC handsets support is the model number. They are very easy to understand. For example, let's take the HTC Desire. Unbeknownst to a lot of people, there are actually two models. The A8181 is the Asian/European model, with 900/2100MHz support. The Telstra model with NextG support is the A8183. There is also an A8182 model from Canada, which will work on NextG but is very rare. In general, if the model number has differing sets of double figures, it's 850MHz compatible. Some other HTC examples:
Desire HD: A9191 (Asia/Europe), A9192 (Telstra)
7 Mozart: T8698 (Telstra)
HD7: A9292 (Asia/Europe)
Touch Pro 2: T7373 (Asia/Europe) T7381 (Telstra)
Touch HD: T8282 (Asia/Europe) T8285 (Telstra)
HD2: T8585 (Asia/Europe) T9193 (Telstra)
Snap: S521 (Asia/Europe) S523 (Telstra)
Desire S: S510E (Asia/Europe) S510B (Telstra)
Incredible S: S710E (Asia/Europe) S710A (Telstra)
EVO 3D: X515M (Asia/Europe) X515A (Telstra/Rogers)
Sensation: Z710E (Europe/T-Mobile) Z710A (Telstra/Bell)
Wildfire S: A310E (Europe/Asia) A310B (Telstra)
One X: S720E (Europe/Optus) S720B (Telstra)
A couple worth mentioning are the Google Nexus S and the Desire Z (also known as the T-Mobile G2). Note that no Australian Nexus S (i9023) will support 850. They are all old Vodafone stock, as Vodafone were the only stockist within Australia (before the 850 network existed). To get a NextG compatible unit, you'll have to keep your eye out for the AT&T model (i9020a). This usually means importing from the US or Canada, although you will find the odd one floating around here. Same goes with the Desire Z (you'll need a Rogers/Bell unit from Canada - A7275).
A lot of older HTC phones (TyTN/TyTN 2, Hermes/JASJAM/838 Pro) support quadband 3G, so they are also compatible. One known exception to this is the HTC Tattoo (A3288). This phone does not support NextG. Also, O2 models of HTC phones generally do not support 850 MHz. Always ask first!
(2) Nokia. Great phones and heaps to choose from but generally the hardest to determine. Not any longer! The easiest way to tell with Nokia phones is the model designation. There are still an alarming number of sellers who don't advertise this, but it is vital when listing Nokia phones. There are generally three 'revisions' of older Nokia handsets. Funnily enough, they are -1, -2 and -3. '1' handsets (E71-1, E52-1, N97-1, N95-1 etc.) are the Asia/Europe models, with just 900/2100 MHz 3G support. For NextG, you need to look for the -2 models (E52-2, E75-2, N86-2) or the -3 models (E71-3, N97-3). '2' models support 850/1900 MHz 3G. These are referred to as 'NAM' handsets, or North American handsets. '3' handsets are 'worldwide' phones. They support 850/2100 MHz 3G. Notable exceptions are the 6120 Classic (which was one of the first decent NextG phones and still quite capable), and the 6700 Slide.
Nokia handsets such as the C-series and X-series use unconventional model numbering systems that don't always reflect which bands they support. It always pays to double-check with Nokia listings. Even newer devices, such as the E7, N9, 808 PureView etc. use a pentaband 3G radio, meaning that as long as they're SIM unlocked, they'll work on anything, worldwide!
Note here that Lumia handsets are not pentaband, and will require some research before purchasing.
(3) Sony Ericsson. This one's easy, just look for an 'a' after the model number. Examples: T715a, C905a, Xperia X10a, Xperia Arc LT15a. Some newer Sony handsets have actual names like 'Satio', 'Aino', 'Naite' or 'Yari', but just look for the 'a' (ask for the model number if it's not advertised) and you're laughing. In case you're wondering, the 'a' means they're American handsets. See a trend forming?
(4) LG phones with NextG compatibility usually have an 'F' after the model or sometimes within the model name. Examples: Viewty Smart (GC900F), Arena (KM900F), Xenon (GR500F), KF700Q, KF390. Exceptions include older models (which had a 'T' before the model number) such as TU550, TU720. The LG Nexus 4 is compatible with all Australian 3G networks.
(5) Samsung phones generally have a 'T' after the model (most Samsung models either start with SGH- or GT-, this is not the 'T' you're looking for). Examples: Preston (S5603T), Omnia 2 (i8000T), F480T, S6700T, Galaxy S (i9000T), UltraTOUCH (8300T). There are exceptions, notably the cheaper handsets including A411, A561, C5220. Newer Samsung handsets like the Galaxy S2, Galaxy Nexus, S3, Note, Note 2 etc. use a quadband 3G radio (once network unlocked), and will work across all Australian networks except 4G, unless you have one of the '05' models (eg. i9305, N7105, N7005 etc.)
(6) All ZTE phones sold within Australia are NextG capable. They are 'Telstra' phones, self-branded. Examples: Tough T90, T165i, Telstra Bubble Touch, F158, F165.
Other manufactuers vary wildly, it's best to check with the seller on supported 3G bands and grab a model number just to be sure. Naturally, any phone with a NextG logo on the box (provided the box is original to that particular handset - make sure the IMEI numbers match) will be compatible. Any AT&T phone from the US which supports 3G will be compatible (including their newer '4G' handsets - a bad misadvertisement by American carriers) including the Samsung Captivate (Galaxy S for AT&T), HTC Vivid 4G (Velocity 4G), Motorola Atrix and Nokia Lumia series. Some Telus Canadian handsets (such as their Motorola Milestone, Desire Z, Incredible S) are compatible, as are some Rogers and Bell handsets (the LG GW620R Eve is a good example). There may be other issues to overcome with these handsets (a new charger or Australian adapter will usually be required - you will also need to unlock them for use if they don't ship unlocked) so be careful and ASK first!
Be very careful of importing non-AT&T/Canadian phones. Steer clear of T-Mobile, Vodafone, O2, Orange, 3 etc. as they all use 900/1700/2100MHz bands and won't work on NextG. If you're importing, always always always (!) ask the question first, even if it's stated in the listing. That way you have it in writing. If the seller can't tell you, ask them to find out (definitely) or withold from bidding/buying. Save yourself the hassles!
One final point. Any phone with a 'blue tick' will be NextG compatible. Telstra use the 'blue tick' to highlight which phones perform better in rural areas. From my experience, the ones to beat are the Nokia E51 and E52, Nokia N95, Google Nexus One and Telstra handsets with extendable antennae (F165, T165i).
I hope you have found this guide useful. The aim is to eventually see a greater number of sellers advertising model numbers, designations and correct network compatibility. If you have any questions, would like to see something added or are still unsure about a phone you're looking at after reading this, feel free to send me a message. I'm always happy to help out.
Take care and good luck! Feel free to message me anytime with your queries, and thanks to those who have!