Do the internals have any serial numbers on? Call his bluff and ask him if he can explain why the serial numbers don't match the ones you sold.
Stuff like this is why I stay well away from ebay. Bought a phone once - described as unlocked/not tied to any network in the description. It arrives, its locked to a particular network. Seller tries to deny saying its unlocked even though its in the description and gets shirty but makes the mistake of mentioning that he works in a phone shop. I put 2 and 2 together using his location and this info and manage to figure out where he works and advise that I'll be paying a visit shortly. (I'm nowhere near - its the other side of the country)
He panics and gets the provider to send the phone unlocking code by the end of the day.