So at the beginning of the game (when you wake up) go outside and open your trunk to the car. then go inside and get the tires and whatever has the most value and put it in the trunk. Then at a gas station you can sell it like you would be buying something (clicking the register with it in your hands). If you hit " go home" or some thing like that in the pause menu, you keep the money and the tires are there again at your house to repeat the process. you can also carry stuff with you while driving and you can buy a rack for on the top of your car to take more stuff and hold boxes.