If a buyer never registers a car you sold them, it’s likely the state the car is registered in will recognize you as the legal owner of the vehicle. This can cause problems if the vehicle is then involved in parking or traffic violations. You can’t force someone to register the vehicle, but you can take steps to ensure you have the legal documentation to prove you sold the vehicle and transferred ownership. This could include the signed certificate of title, as well as a signed bill of sale. In some states, it could be as simple as reporting the sale of the vehicle, which you might be able to do online.