If the seller was the original purchaser of record and activated the phone with a carrier, then yes, they could mess you up. Carriers have been known to blacklist phones when reported stolen by their original owner of record as basically they simply take the word of that person since they are on record as having originally purchased the device new. Sellers on Craigslist and eBay have reportedly sold stolen phones which were reported by their owners to their carrier, so the naive buyer is stuck with a blacklisted phone. The USA has a universal blacklisting service in place as do some other countries. Once blacklisted, US carriers cannot remove a device ID from the database. If you buy used, you buy as is with the ever present caveat of buyer beware. Ain't-theft advances like Activation Lock and universal blacklisting do help owners, but they make the used market much more of virtual minefield.