OK, so, you pay for the car, right? After you paid for it, it is your possession. So, if the seller wants to tell you what to do and what not to do with the car AFTER you bought it from him, I'd tell him to go fish, OR sell you the car for a lower amount of money, because he restricts you in what to do with the car... At least, that is the way I see it.
-Joost-