To duplicate items, equip the item to be duplicated. You must only have one of that item. Get into a Free Battle with a Harpy, which can be found in Bandit's Forest. Have Labyrinthia create a Replica in front of the party, and wait for a Harpy to use "Steal Item" on the Replica. Have the real Labyrinthia use her copy of the item, then finish the battle. You will now have 255 (appears as 0%55) of the item to sell. The Gella Card is worth 380,000 each time. Repeat this process as many times as desired. Note: Other items in your inventory may become inaccessible in some menus. To fix this, save the game and reload it after you are done selling.