Optional Item: Boomerang
Table of Contents
Make a deal with the Item Trader
Magnifying Lens Required

After you have completed the Trading Sequence, travel to
Use a bomb to open the cave.
If you have the Magnifying Lens, you will see the Item Trader, a Goriya, waiting for you in the cave. If you do not have the Magnifying Lens, the cave will appear empty.
Speak with the Item Trader to make a deal. He will trade you a “good item” in exchange for an item you have. The “good item” is the Boomerang.
The Item Trader will trade a surprising number of things for the Boomerang. The only items he will not accept are your shield, the Power Bracelet, Bombs, Magic Powder, or the Bow. Anything else is fair game!
The best item to trade is the Shovel. This is especially true if you have already collected all of the Secret Seashells. Late in the game, the shovel is no longer required to proceed.

Unfortunately, the Boomerang isn’t free. You have to trade for it. 
The Boomerang is far more useful than the Shovel will ever be.
The trade is not permanent. You can return to the cave at any time and speak to the Item Trader again to reverse the trade. He will return your item to you, and you will give the Boomerang back to him. You can trade as many times as you want.
Purchase your item back from the Item Trader
Switch Remake Exclusive

The Nintendo Switch remake added an additional option. After you trade for the Boomerang, you can return to the cave to purchase your original item to permanently return it to your inventory without giving up the Boomerang.
The Item Trader sells your item back to you for 300 rupees.