It works a lot like the Resident Evil glitch. One person gives someone their gun, and then does not save and exits to the dashboard, signs out or turns off their Xbox. Then the other person picks it up and when the first person signs in, it they will have the gun as well because they didn't save. This can help with money as well because you can get a lot of money for selling certain guns.
Easier Way -
1. Get a friend to invite you to their PRIVATE GAME (you dont want people stealing your stuff - you will see why later)
2. Then go to Firestone as there is a shop there. Then when outside the shop, drop all of your stuff, a bigger inventory means more money. Just drop it all in the same pile and then exit without saving.
3. However get your friend to stay in the game as the host. Then boot up the game again and get him to invite you and you will see that all of your belongings are still on the ground in the same place you left them and remain there for a long time (I think until you move to a different location). You can either then drop all your stuff again and do the glitch again, or sell all the stuff you are holding now and then pick it all up again from the pile. Just remember to save after you are done.
Glitches That Unlock Achievements 