Selling cars on GTA 5 Story Mode is pretty simple. Start by acquiring the car in question, and driving it out onto the open world. Then, navigate to any Los Santos Customs location, which can be located on your map by the spray paint icons. Drive to a Los Santos Customs garage and drive your car inside. Once it all loads in, a series of options will appear on what you can do with this car. Find the 'Sell Car' option in this menu, click it, and some cash (equal to the value of the car) will be deposited into your account. This is a neat way to quickly earn more money in the game. In short, stealing the fanciest cars that you can find, fixing them up a bit, and re-selling them. Realistically, you can usually find the most expensive cars in the fancier parts of Los Santos.