I forgot to say this earlier, your best bet is probably ebay or similar. My girlfriend sold her her iPhone 4 the other week when we switched from ATT to VZW. We tried flipswap and a few other sites. They were going to give her about $300 maybe, and sold it on ebay for over $400. Ebay does take 7%(I think) of the selling price, but imo, its the easiest way to sell something like this.