I've sold a number of smartphones on eBay. In fact, I'm selling one right now and I'll be selling another one next week.
If you're thinking about buying a used smartphone on eBay (or craigslist, or any other used seller), make sure to ask about the phone's history. The last thing you want to do is buy a used phone that's been reported as lost or stolen. Some sellers will indicate that their phone has a "clean ESN." This means that you should not have any problems activating that phone on your account.
GSM phones that use SIM cards may be locked, unlocked, or be limited to some international markets. It's important to verify that the phone will work on your network. Otherwise, you may find yourself reselling the phone on eBay.