1. Equip a valuable, useable item that you have only one of (sell down to one) to Labyrinthia. (e.g. Gella Card)
2. Get into a Free Battle that contains the Harpy enemy (Bandit's Forest, et al.)
3. Have Labyrinthia create a Replica out in front of the party, and wait for a
Harpy to use Steal Item on it.
4. Have the real Labyrinthia use her copy of the item.
5. Finish the battle.
You will now have 255 (appears as 0%55) of the item to sell. With Gella Card this is worth 380K each time you do it, and can be repeated as often as you like. It can be done as early as 1-4, when you first get access to Free Battles.