I recently upgraded to an iPhone 3GS, and now need to sell my original 16GB iPhone (2G). On eBay, a mint-condition jailbroken 16GB iPhone 2G sells for $300-$350, while iPhones that look used are going between $200-$300. Assuming I managed to get $300 for mine (unlikely, even though the screen is perfect -- it looks used), I would pay approximately $21 in fees, plus at least $10 insured shipping, netting me around $269. For my phone, FlipSwap offers $235 in the form of an Amazon gift certificate, or around $223 in cash. I've decided to sell via FlipSwap / Amazon gift certificate because the $34 difference isn't enough to make me go through the effort of listing on eBay and risking a sale price lower than $263, which the break-even price at eBay after fees. I'd also risk an international sale, which comes with its own set of issues, like risk, and time to actually go to a post office. Also, I'd have to either wait for a 3.0 jailbreak tool or downgrade the firmware from 3.0 to 2.2.1 for jailbreaking before selling it on eBay. More effort.
FlipSwap seems like the best deal, that is, unless someone reading this wants to buy my iPhone for more than $235 + shipping. :)