You’ve gotten a dilemma.
The one that bought it ought to have ensured that the earlier proprietor had already launched it from Discover My.

That they did not leaves two potentialities.
They forgot to examine.
It was stolen.

You may contact the earlier proprietor and ask them to launch it from Discover My.
They could.
They may not.
They may be pleased to seek out their stolen items & need it again.

Apple will launch it for you with proof of possession. This isn’t the identical as proof of buy. You solely have proof of buy. The individual with it locked to Discover My has a greater declare.

I believe, as you acquire it from Amazon, your most secure guess is to ship it again to Amazon & get your a reimbursement.
This isn’t essentially ‘truthful’ but it surely absolves you of accountability in coping with a doubtlessly stolen Mac – whether or not stolen or not you might be nonetheless held at ransom by the individual it’s nonetheless locked to. They’ll erase it at any time.
Amazon will simply offer you your a reimbursement, if the vendor will get awkward.