There is some binding of the purchased devices to the AppleID.
If the laptop really is tied to the profile, your seller, let him come in supportprofile.apple.com
After logging in there he will see a list of all devices tied to his AppleID. There he needs to check out this laptop.
After that you just go for this link, log in with your AppleID and tie this thing to your profile by entering its serial number.
However, I'm not so sure that this will help, but worth a try.
I father-in-law in Moscow, bought a couple of years ago, a new Mac Mini, it was tied to the profile of the person that bought it to the Apple Store and this man could not find for Mac(bought from the huckster's already here). At the same time on this computer all OSes were put without problems.
Apple will not release for you this laptop from another profile for security reasons. However, they also say that such a reference to anything in principle is not affected when using. Don't know, maybe lately something has changed in this policy. Either the cause of your problems is not in this bind, and in some other. Perhaps somewhere in your iCloud, the seller must detach the laptop, or somewhere else.