I put mine up on CL and figured I wouldn't get more than $250, so I started at $300. I received 8 emails in 20 minutes and someone picked up within an hour. I had a 16gb 3GS, with a small crack on the bottom, near the home button. I checked later in the day and people had theirs up for $400-$500.
My co-worker posted his on eBay, with a Buy-it-Now option, for $300. His sold literally in 5 minutes. Not exaggerating at all.
I started thinking I may have under-sold mine. Either way, I'm just glad I wont be out any cash to upgrade.