I think this would be incredibly risky. The seller of the directly mined bitcoin could easily sell the private keys twice (or more) or they could sign and broadcast a TX out of the address at a later time, and there would be nothing the buyer could do about it because of how difficult it is to determine where a TX came from.

A possible alternative would be to sell bitcoin whose inputs are from the block subsidies for a presidium (you would actually send the bitcoin to an address that the buyer controls). My concern about doing this however is that it would put the fungibility of bitcoin into question.