Aight let's say like this.
To sell a car you have to own it, or find it unowned, and then you can advertise it. If you sell a car you own, then the money you sell it for is money to set it unowned for the person who wants to buy it, it's not the value. Usually most people pay more for cars you do own, not so much for unowned ones, depends on which car it is, too.
Edit: Fabio posted while I was typing D: