Give him $800 and pull it apart (you WERE going to pull it apart to check, right? )
You find as advertised, pay him the rest. You find otherwise, keep it. Just because the parts are in there doesn't mean a bore ain't cracked, half the rod caps are backward, a skirt isn't scuffed or broken, a standard piston in a oversize bore, etc....
If he doesn't like your proposal, remember that deals are like buses: another will be along shortly