Choose a high-priced item to sell, then use the "Item duplication" trick. Sell all of them except one, then repeat as many times as desired.
Go to the Town Of The Ancients and talk to the child two stories up the grass hills. He is back behind the stairs. He will tell you that he will give you a prize if you ride around the continent. Leave town and go east a bit. Get a Chocobo and ride around the outside of the continent. Go back and talk to the child to get your rewards. Sell them, then repeat.
