This glitch allows you to duplicate another player's vehicle, which you can then either keep or sell . First, request a job from Gerald while you are in another player's vehicle. Use the other player's vehicle for the mission, and make sure you are driving it when the mission ends. You should then enter free roam mode, and still be in the vehicle. You can then go to Los Santos Customs and try selling the vehicle. If you cannot sell it at that point, add a tracker to it and try going back later to sell it. If you want to keep it, add a tracker to it and insure it.