The bottom line is, that you don't know what the provenance of those phones is any more than we do. You bought it from a "guy" who buys them as lots at "auctions"? So, where did they all come from? I don't know. Do you?
Whether you do or not, whether or not they are totally legit, or a broken device from your friend's kid, who doesn't know what password he put on a device, at the end of the day, there is only one answer:
You must either get the Apple ID and password that is signed into Find My iPhone, and use those to turn it off and remove the Activation Lock.
If you cannot get the original Apple ID and password, then you need the ORIGINAL sales receipt, a photo ID, and the device, which you can then take into an Apple Store, and they should be able to remove the lock for you.
The provenance of the device is really irrelevant here. The rules remain the same regardless of where the phone came from - either turn off the Activation Lock with the Apple ID and password that were used to put the lock on, or present the ORIGINAL sales receipt along with the device and ID to see if Apple can remove it for you.
You can do one or the other. Passing no judgement here - just telling you what you need to do to get it unlocked.
GB