This GTA Online money making glitch and will make you over $1,000,000 (yes, A MILLION) in less than 2 minutes by reselling a fully upgraded Truffade Adder.
1) Buy the Truffade Adder and buy as many upgrades as possible at Los Santos Customs (LSC). Any car will work, but reselling the Adder will make you the most money.
2) Leave LSC, then Pause, Online, Options, and set Spawn Location to Last Location
3) Switch to single player, start a New Story
4) Play GTA Online, INVITE ONLY SESSION
5) Drive your expensive car to LSC, park somewhere close
6) Steal any random car, drive it into LSC, choose Sell, but STOP when it says "Are you sure you want to sell this vehicle?"
7) Press Pause, launch the Xbox Games Stores
8) Back out of the Xbox Games Stores, and you should be in the stolen car in LSC.
9) Choose Sell again, STOP when it says "Are you sure you want to sell this vehicle?". Press Pause, launch the Xbox Games Stores/Playstation Store, then back out of the Xbox Games Stores
10) You should now be in your personal vehicle outside of LSC. Drive into LSC
11) Sell your vehicle, quickly Pause and launch the Xbox Games Stores
12) Back out of the Xbox Games Store
13) Repeat step 11 & 12! Profit!
If you get the "We're not buying cars right now" error message from Los Santos Customs, wait 48 minutes (2 days in game time) and try again.