This trick requires two players in Multiplayer mode. Join a game with your partner. Go into a trade screen, and enter the screen that displays the amount of gold, but do not choose an amount. Set the slider all the way over to the maximum amount. Once it is at that position, rapidly tap A. When you take your gold back, it should have doubled. Repeat this as many times as desired.
Use the following trick to duplicate any item (armor, potions, spells). It requires Multiplayer mode. One player must save his or her character by dropping any non-valuable item on the ground. Have that person sell all of his or her gear (items, potions, spells, or whatever you wish to duplicate) to the nearest vendor. Then, have the partner buy all of the previous equipment. Once the partner has all of the equipment bought, have the first player go to his or her Xbox 360 Dashboard (thus not saving his character after the selling). Both characters should now have copies of all the items. This is especially useful for getting weapons that do one million damage, armors that are stacked thousands of times, and massive amounts of stat point potions. For example, duplicate a level 10 Anathros and add it to your original level 10. It will now be at 20. Repeat this to get 20 to 40, and so forth.
This trick requires the Disarming Shot skill in Multiplayer mode. Have your partner equip and pull out his or her weapon. Use Disarming Shot to "steal" his or her weapon. Once the weapon is dropped, have your partner go to the Xbox 360 Dashboard. This will not save his or her character, and you will have a "copy" of his or her weapon. This is very useful if you have many weapons you want to duplicate. Just have him or her equip it, shoot them, equip a new weapon, shoot them again, and repeat.
