After the battle, when the player checks the inventory, if done correctly there should be a 55 next to the name of the weapon that was just stolen. As long as this is never moved from the first slot (via manually or through Sort), or no extras are bought, the player can keep selling that weapon infinitely. However, the player can only sell one at a time, and doing this trick with items will force the infinite item to never be usable outside of battle unless sorted.