For those having problems, give this a read and you should be fine. When you first upgrade the car, DO NOT SELL IT RIGHT AWAY. Go to the store. Pause and push L1 X X. You then back out and will be back with the bike. From here you can go to Los Santos Customs and sell it over and over again. Enjoy the glitch!
GTA 5 online how to get money fast using an unlimited money glitch