In the UK if the car was bought under the Hire Purchase Act 1964 anyone who purchases the vehicle and is not in the motor trade and is not aware of the HP agreement holds "good title" to the vehicle. This means that somebody selling a car they have under HP is still liable for the payments and the buyer legally owns the car. If the car moves through a trader while under HP the buyer is still ok.

I suspect if you buy it and the person selling the car tells you it's on HP and that they will pay it off using your money and then doesn't pay if off you are stuck 'cos you are not "iinnocent" of the HP agreement.

Gareth