From the bank's point of view - you own the car. You are responsible for making payments and if the payments are not made - they will file a case against you, it doesn't matter what agreement you have with this third party.

As for the contract between you and the third party - is it enforceable? I don't know, but you should consult with a lawyer. In any event, if he skips town or stops making payments - you are liable to the bank no matter the outcome of any dispute you have with the third party.