First make sure they are even allowed to own a domain like that, I believe some franchise agreements prohibit domains such as yours. Do you know how many car deals are generally produced from your domain per month? If you know that, then you should use that info to generate a price. Traditionally one can expect to pay a year's worth of profit when buying a business, so apply that principle to your site.
I don't think you need to give it away or undersell it for any reason. You own something with value, it's no different than Autotrader, cars.com, etc. The concept of expecting a profit as being something undesirable is a disturbing trend nowadays, I'm frankly surpised to hear similar attitudes coming from some car guys here.