All civilian can sell vehicles. When you get into any vehicle that is sellable, a message will show up on your screen showing how much you can sell the vehicle for. 

After you successfully steal the vehicle, drive yourself to the nearest Vehicle Exporter and press the Sub-Mission Key (2) or type /sellvehicle when you're inside the checkpoint with the vehicle to sell your vehicle. The money you receive depends on the damage on the vehicle and the automotive market rate. 

Selling Vehicle Commands:
/sellvehicle - Sells the vehicle you're in at the Vehicle Exporter