The same as this question ( http://gadgets.stackexchange.com/questions/9/what-do-i-need-to-do-to-sell-my-iphone-3gs ) except with the UK in mind.
I have a 16GB iPhone 3G and have recently upgraded (we don't need to argue if it's an upgrade :p) to the iPhone 4. What's the recommended way to sell it in the UK? It has one small crack on the back near the vibration switch so it probably doesn't count as near new (depending on your definition of near).