Best Site to sell an iPhone?

I'm about to transfer to Verizon and want to sell my AT&T iPhone to cover the cost to get the Verizon iPhone. I'm trying to decide where to do this. I have sold on Ebay before and was considering that, but worry about the way that sellers are not protected as much as buyers are. Everything I've sold on Ebay hasn't been as much as this, so I'm worried that if I sell this to someone and they say "Oh, I never got it, refund me", I'll be out 200+ and they have the phone, or they say it's damaged when I sent it perfectly fine, or something that would end up screwing me over. Is there a good website out there that will pay at least 200 for an iPhone 3gs white 16gb? A bunch of those cell trade in sites are offering sub-200 for it.