I had an iPhone 4 with AT&T. I got it unlocked and sold it a few days back. The buyer just contacted me, though, and said that when they tried to activate the phone they received the message "We’re sorry, but this is a serial number for a product that has been replaced." Looking online, I see all sorts of things about people saying that message means the phone has been stolen or something along those lines (which I'm sure makes the buyer think very highly of me), but it occurs to me that I did actually have the phone replaced at an Apple store a while back after I accidentally trashed the original one. If you had your phone replaced by Apple, are you not able to sell that phone later on? Just trying to see if there's anything to do before sending a refund and getting the phone back.