bypass activation lock

If activation lock could be bypassed, it would be pointless for Apple to even offer at all. It is intended and designed specifically to prevent anyone from activating and using a device unless they know the AppleID password used to sign it into iCloud.

Without that information, or the cooperation of whomever knows that information, the devices are useless to you. Apple will not help at all when it comes to personal or private sales of personal devices that were not properly prepared for sale by the owner.

Nope.

Since the "old owner" now seems to be blowing you off, there I a very real possibility that the devices are stolen.

Turn them over to the police and give them all the information you have on the seller.

No