well, for starters, there is the much known W-Item glitch, where you select an item, and then the next one, but cancel before choosing your target for the second item, thus leading the game, who thinks you're just using regular item command, to put back one of the first items, adding one to the inventory. Thus you could go to the chocobo sage, buy one sylkis greens, go into battle, duplicate it 98 times, and sell those 98 for big profit and go do it again.