About selling a brand new iPhone on eBay?

So, I plan on selling an iPhone 4 on eBay, brand new, never out of the box, but I'm a little weary to. If I sell a brand new iPhone 4, locked, does that that means the person who buys it will need to bring it to the AT&T store to get it activated? If so, would the buyer have to sign up for a contract while there, which defeats the whole purpose of buying on eBay and not AT&T? Or, can they just put in their SIM card and go?