It's a field I've dabbled in, broadly speaking, but I haven't found the (probably academic) networks I've been expecting.
Three tips! 1. Check the prices on eBay of all the free/low cost phones your carrier is offering. At the moment the iPhone 4 8GB (Black) edged out others, but that could change. Particularly if a lot of people do what I did, although the market seems to have a normal supply at the moment. 2. Don't open the sealed phone box, it's will sell for more if it's "unopened, factory sealed". 3. I don't have a ton of evidence to back this up, but I included photos of taking the phone box out of the AT&T shipping box along with all the AT&T papers, which I believ gives bidders greater confidence and will raise the auction price.
I know this is fairly obvious to a lot of people, but I thought it might be helpful for people that only consider getting free phones on contract.