You have a dilemma.
The person who sold it should have ensured that the previous owner had already released it from Find My.
That they didn’t leaves two possibilities.
They forgot to check.
It was stolen.
You could contact the previous owner and ask them to release it from Find My.
They might.
They might not.
They might be happy to find their stolen goods & want it back.
Apple will release it for you with proof of ownership. This is not the same as proof of purchase. You only have proof of purchase. The person with it locked to Find My has a better claim.
I think, as you bought it from Amazon, your safest bet is to send it back to Amazon & get your money back.
This is not necessarily ‘fair’ but it absolves you of responsibility in dealing with a potentially stolen Mac – whether stolen or not you are still held at ransom by the person it is still locked to. They can erase it at any time.
Amazon will just give you your money back, if the seller gets awkward.