In mission "Tanker Commander" Mr. Wittaker told you to sell him stolen rigs. Because of unknown reasons Rockstar didn't include this opportunity to the game.
Your bad, Housers! But i'd fix it, don't worry.
Now you can sell stolen tankers to RS Haul, with cabs. This is an easy way to get money, 5000$ for a ride, just like in original mission.
Opportunity opened after "Tanker Commander" mission.
Enjoy!