Note: An unpatched game is required. Select any car and purchase 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 just 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 while the one that you purchased should still be there. You can repeat this as many times as desired. Note: if you have already purchased the best upgrade, this glitch will not work--instead it will sell the best upgraded for sell price.
Note: An unpatchedgame is required. Level up five Porsches (any, even the R class). If you still have the Chrysler ME-Four Twelve, select it and go to "Upgrades" then "Tires". Choose the Perrell tires and they should be 50% off, costing about 17,000 CR. Buy them, then sell them for 21,000 CR.