What Peter brings up is a not an uncommon problem users have with second-hand purchases...
1. A former owner who has not properly prepared the iPhone for resale and is not locatable
2. A stolen item and the seller, for obvious reasons, is not locatable
3. A buyer who is now stuck with an unusable iPhone, and Apple will not get involved since they created the protection system for a very good reason.
Follow Peter's advice and first find out the carrier the iPhone is locked to so you can activate and see if it is not locked to an Apple ID, also.
They will simlpy ( MAY) tell you the carrier who locked the iPhone to their network
Then you determine if that Carrier offers an unlocking service and what their terms and charges are for unlocking the iPhone
BIG IF DO they offer unlocking ???
Apple cannot unlock a Carrier locked iPhone ,they have no legal right to do so