Legal Question in Real Estate Law in Missouri

The contract is what the court decides it is. If there is a writing that appears to be made and executed by both sides, than the Plaintiff (former owner) has at least made it submissable case as to the terms of the contract being what are in the writing. You may or may not be able to get oral terms into evidence. If the writing appears to be only executed by one party, as you say the one on your vehicle is, then the court will be more likely to consider and believe evidence of oral terms that contradict the writing. . If the writing appears to only be executed by the party presenting it (in your case the seller) the court may be less likely to believe it contains all the terms by which the defendant (you) should be held to.

Without knowing what the writing the seller has says, or what the other facts of this case, it is nearly impossible to determine if you will win this law suit. There is almost never enough space here to provide all the relevant information about a case. You should consult directly with an attorney in your area, who can review the documents and discuss the case with you, if you desire a more definite guess than "maybe" you can prevail in the law suit you described here. Many attorneys provide a free initial consultation.

Good luck