You will have to cross the border and sell the car in the USA, assuming that it is registered in the USA.
Before you exit Mexico, be absolutely sure to go to the 'bancercito' and have them, not you, remove the sticker from your vehicle and give you a receipt, which you will keep forever. That will absolve you of your bond and other responsibilities, including the possibility that the car may be returned to Mexico and could become involved in an accident/injury/death for which you could be held responsible if you don't get the sticker removed properly; serious business. This will also permit you to bring another vehicle into Mexico, should you want to in the future.