So i bought a brand new S5 from Swappa. The guy purchased it with a new postpaid account (2 year agreement). He is sending the receipt for the device with the phone. He said it wasn't purchased through Next and said he will cancel the account and settle up the early termination fee with AT&T once I am using the device. How do i know this will not become blacklisted? Did I make a mistake here? Any tips or pointers on what I should do to protect myself since the deal has already gone down?