After you complete the mission where you steal a car from the docks via a crane with Cesar (after buying Wangs), the crane will be unlocked and you will be able to export cars. To export cars, drive a car on the wanted list (at the end of the docks) to the crane and activate the crane. Lift the car into the checkpoint and you will be paid. You get more money if the car isn't damaged. To import cars, stand in front of the wanted list and press RETURN.