Contact an attorney in your area for help. Normally when a vehicle is sold, the title is transfered, and a security interest in the car is held back to secure payment. Title is transfered for a number of reasons, the most important being that most people do not want an unrelated driver driving their car around. In your case, the car has not been transfered, and so the car is still yours from a title standpoint. However, the purchaser does have rights based on your oral contract, and an attorney can help untangle the situation.
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It's your car. Do you have the keys? Sounds like the contract would not be enforceable by either of you. If you do not disturb the peace it sounds like you could get in it and drive away, but you may have trouble with the alleged owbner of the vehicle getting mad and calling the police. Since you are the only person on the title it sounds like they would have to go to civil court to sue you if they think hey have a claim. I would talk to an attorney before I tried self help, or at least let the police know hat you are contemplating before you do it.