Note: This trick will only work if your game has not been patched through Xbox Live. Select any car. Buy the upgrade second to the last. After buying it, reinstall the stock part, and there should be a blue check on the purchased part. Go to it, then press RT + X as you are selling the part you just purchased. It should move over to the one on the end. You will receive the money for the one on the end, and the one that you purchased should still be there. You can repeat this as many times as desired. Note: If you have already bought the best upgrade, this glitch will not work; instead, it will sell the best upgraded for the sell price.