There are some basic rules which can really help ensure you do not pay too much or get ripped off when buying Australian Coin & Banknotes on eBay.
1. Avoid buying off anyone who does not provide quality photos that clearly show the item to help you make an educated guess on the grading. There have been many occasions with bank notes especially where the grade has been stated as UNC but upon arrival has been more like an EF or in some cases even lower. With many sellers falling back on "grade is my opinion only" which is certainly fair enough everyone grades things a bit differently but returns should allows be allowed under circumstances like these which brings me to my next rule.
2. Always try to buy from sellers who allow returns, at least this way instead of holding back on bidding out of fear of an item being nothing like described you can happily bid with confidence knowing you can get a refund if needed. In saying that you do need to make sure if you are returning anything especially bank notes which can be easily damaged you package them very safely obviously if you send back a lot of notes that get screwed up because you send them in a normal enevelope with no protection you won't be getting any money back. You always want to have a good look through someones feedback and actually read the comments, it can really reveal a lot about what they are like as a seller and a person to deal with. On occasion I have gone to buy something and seen they offer refunds if you aren't happy and thought you beauty but after having a quick look at their feedback saw comments such as "took 2 months to refund money" or "hard to contact, doesn't reply to email". Even if someone offers refunds doesn't they will make it easy so it is always worth having a look through the feedback.
3. I find bank notes particularly difficult to buy online as you really need to trust the seller because photos really can't tell enough. For example you can not tell if a note has been flattened or treated from a photo, not to mention that a note can bear quite a number of small crease or lines that don't show even in a great photo. So picking someone who is really descriptive of there items for example mentioning any faults or giving a more accurate description than just a grade is always a good idea and again looking at someones feedback can reveal a lot and the more expensive the item you are looking at buying the more time you should putting into checking them out.
4. Catalogue values are a really useful guide but do not take them as gospel, for instance an Uncirculated catalogue value is meant to reflect the value of a coin or bank note that strictly meets the standards of being uncirculated. But there are many items that grade uncirculated but may have detracting marks giving it a lower value for example an Uncirulated coin with only 10% lustre is not as valuable as one with 50% lustre or for bank notes a note with a light smudge mark is not as valuable as one without even though they both grade uncirculated. This again really comes down to sellers being descripitive with examples of have given can look identical in pictures and so you have to rely on a seller to give you all the information you need to make an informed decision. Also affecting value is if a coin or note has been artificially altered in any way which can involve anything from cleaning, chemically treating, trimming, stretching, flattening and I am sure there are plenty of other things they are doing now days. In almost all cases items which have been altered are not as vaulable those that have't and in many cases it is impossible to tell without physically having it in your hands.
There are plenty of good sellers on eBay it is just a matter of sifting through the 'Dodgy' ones.
If you have any questions please don't hesitate to contact me.
Best of luck,