Note: This doesn't work on high priced or third party cars. Buy an old or new junker from the auction but do not repair this car. Just take and buy a new car door from the catalog. Put the door or whatever part on the car and auction it. This will generally make anywhere from $78 to $1,000 profit.
I f there is a "_______.car" or "_______.spec" in your folder, you have to remove it. This will glitch up the game and often only show a car with no parts. No one will bid on it, and it is easy to get for $125 but you must remove this file before playing the game.