Max out your item capacity, this requires 100 items. Win another item through a tournament. The game will ask you what you want to do with it, you can either sell it or exchange one of the items you have for it. Choose to sell it. The game will tell you how much it's worth. Confirm. However, the calculation of how much it's worth is not done properly, the game awards you at least 100x the item's value, which almost always instantly maxes out your money. This way, you effectively have infinite money.