I'm not sure buying stolen laptops is really the direction we want to go in here.
Found the former owner of the laptop where the board came from thanks to the corporate/user name displayed on the password prompt. It was a small IT company.
Contacted them on LinkedIn asking for the password and detailing the situation. Didn't get the password, but the CEO/owner of the company certainly had a surprise.
Turned out one of their guys was selling parts from their laptops/hardware without permission.
Suddenly, the seller on eBay found the message feature and wrote to me, saying I got them in trouble. Shouldn't have ghosted me, then? Just a "sorry, didn't know it had one" and I would've just proceeded to replace the BIOS chip, which was my plan B and it's what I ended up doing.