So you bought what you must have known was a stolen laptop and you are expecting help on this forum. Good luck with that!
If I were you I would delete your post and your user ID on this site and get the hell out of it before the police start investigating. It's never clever to admit to a crime on a public forum.
(Oh, and you'll probably want to ditch that laptop asap. If the legitimate owner has been sensible it can be traced fairly easily.)
Edit: I've just checked a bit further. The fact that the Mac is asking for a PIN indicates that the anti-theft tracking software has been activated. It's probably too late now, so expect a knock on the door anytime soon.