What do you guys make of the fact that the product box he shipped (the Apple iPhone box) lists a different IMEI number than the one on the back of the phone?
Means nothing, just that he got a replacement iPhone under warranty. Unless the seller specifically states it's the original iPhone or a replacement iPhone, it's fair game as to which you get. People replace phones all the time under warranty.
Your biggest concern should be being to have the seller to activation unlock it. Otherwise, file a not-as-described dispute and send it back with a tracking number. ebay will side with you.