I was able to sell my 3GS with no problem on eBay but ran into all kinds of trouble selling the 4. Some dumbass kid won the auction and paid, and then 2 hours later decided that he'd "made a mistake" and wanted a refund. My gut instinct told me to tell him to pound sand, that he could figure out what to do with the phone once he got hold of it, but everyone told me that that would kill my rep as an eBay seller, so I granted the refund, which involved transferring the money back out of my bank acct. Major pain in the ass. PayPal promptly put a hold on my account for several months because of suspected fraud. Grrrrrrr.
I'll give it a try again with my 4S, though. Just make sure you limit the number of countries to which you'll ship (there are some countries that are more prone to generating fraudulent transactions than others) and put whatever other restrictions you need to on the type of buyer you'll accept (# of positive feedback, whatever). I'm also going to set a reserve price on mine because I don't want to have to settle for selling it for some ridiculously low price if it gets lost in the shuffle of all the other iPhones flooding the market. Oh, and make sure you set the auction to end at a time of day when the majority of your buyers are going to be home, like in the evening. I've made the mistake of having auctions end in the morning before and I'm sure I lost out on some buyers because of it..