Have you rung the police yet?
I don't see that it's a police issue. He saw the laptop, examined it, and decided to go with it. At any time he could have bailed, no one twisted his arm.
I agree, it sucks, but I don't see that it's really a scam or a police issue unless the machine comes back as stolen. Caveat Emptor definitely.
If you're going to buy and sell used equipment should have a bootable disc with diagnostics though rather than trust a sellers word for it.