I thought this too at first... but he bought it off FB marketplace. So the seller had a FB account. I assume that if the person he bought it from had stolen it, that person wouldn't have factory reset the device and then put their own FB account on it and then purchased games for it before selling it.

The seller has blocked me on fb

I believe you see the same error page when you're blocked as you see when an account is deleted.

Obviously it could be the case that OP just didn't look into the settings deeply enough and just didn't realize that the headset was tied to a different FB account than the account he purchased the headset from on Marketplace, but I think it's just as likely if not more likely that this is happening because the seller deleted their FB account.