Ok so you may have upgraded or just looking to sell your Mobile Phone? well why not choose Ebay? its one of the biggest auction sites and has alot to offer for selling a range of items. It will give you alot more exposure than a local paper would.
List your Mobile phone under the "Brand" as this will be specific for people looking for your branded handset. From my expereince try and take pictures of your handset, not one sourced from the internet. This way it will give suspecting buyers an exact idea of the condition of the item.
List all features, this can be sourced directly from the owners manual, online reviews and of course the manufacturers website. You can promote your item very well with features like MMS, Bluetooth, speaker phone, GPRS (internet), Camera...the list goes on and on!.
When listing your item, it is also a great idea to promote the fact that you may have the orignal box and manual! this gives the impression you have looked after the item and also have all accessories it came with when purchased.
List and photo additional items like charger, manual, headset, handsfree, leather case etc etc. This will "really" add value to your item.
Another note to selling your item is to note that your handset is either "network locked" or "unlocked". Whats does this mean? it means that the handset may require a certain company branded sim card to use in the phone, ie Optus, Vodaphone, Telstra. (This usually occurs with prepaid handsets.)
Lastly, before you post the item, delete all personal information, like sms messages, phone numbers, addresses, pictures etc! You want to trade safely and protect personal information.
Now you have the basics, Go and sell those phones! its a HUGE market out there for these items and im sure you will sell your phone easily.