In the buy/sell menu, highlight any unwanted equipment. While holding the R2 button, repeatedly press X. This works only on the items in the equipment menu (the shield icon), not the items in the Bag icon. This works in any shop, including the little fast travel buy/sell menu. This glitch may have been patched, and does not affect trophies. Note: The item you are 'cheating' with will be sold and gone when you release R2.