The police should actually help you to find this man, but they are under-resourced and that's probably why they can't devote the manpower to it. Still, ask if they can put you in touch with their autocrime unit because this may already have tabs on the crook who sold you the car. You seem to be blameless here because you not only did all the proper checks, you doublechecked. Your huge, colossal, really stupid mistake was to pay cash on a Sunday. The only safe way to pay for a car or accept payment is inside a bank where the money can be taken from one account as 'cash' and transferred to another account as 'cash'. That way the deal cannot be revoked, but you still have tabs on the bank account the cash was paid into.