This only works after you've sold the Keaton mask to the guard outside of Death Mountain. Once you've done so but before getting the Skull Mask, you can ask to borrow the Keaton Mask again for free. Once you've done so, leave the shop, go back in and ask to trade masks. You can trade the Keaton mask for the Skull mask (he'll say you have to pay, but you won't) for free. It even makes the sound of you giving him the rupees, but you won't actually be paying anything. The same works for the Spooky mask and the Rabbit hood-- you just have to trade the mask that you sold before it.