Can a contract written by private parties have a full legal strength?

If I were you I would request the payment book and send the payments in myself. And you can provide a copy of the check to the Seller each month so that he/she knows you made the payment. Or you can make the check payable to the lien holder and give the check to the Seller and let them send it in. I definitely wouldn't give cash or a check payable to the seller and just trust him/her to pay it. I work in a law office and have seen several people get burned on a deal like this. Just last week a guy came in because he was making cash payments to the seller but the seller was keeping the money. The lien holder repossessed the truck after the poor guy had already paid for over a year.