1. Equip a valuable, useable item that you only have one of (if you have more, sell down to one) to Labyrinthia.
2. Get into a Free Battle in which you fight the Harpy.
3. Have Labyrinthia create a Replica out in front of the party, and wait for a
Harpy to Steal it.
4. Have the real Labyrinthia use her copy of the item and than finish the battle.
You will now have 255 (appears as 0%55) of the item you copied to sell and you can be repeat the process as often as you like.