Name your character one of the following for a boost in statistics. Only certain classes will be able to benefit from the stat boost, however.

While in a shop, highlight an inventory item and press A. Move the cursor to the sell box, then press R, then B to move the item back to the inventory. Then highlight any item you can afford from the store and press A. Move over an open inventory slot and press R to move the item back to the sell box. Press A to sell the store back its own product. Continue to infinity.