Once you have completed the trading quest and earned the Magnifying Lens, return to Toronbo Shores. One screen to the west of where you and Marin had that romantic chat upon the beach, you will find a crumbled wall in the hillside. As with all tell-tale cracks in walls, only one solution will do: Bomb it. A well-placed Bomb will blow that hole wide open.
Inside, but only if you have the Magnifying Lens, will you see a lone Moblin hiding within its cave. However, the creature will offer you a prize for finding it. It will give you the Boomerang, one of the most powerful and useful items within the game. However, this item doesn’t come for free. In order to obtain it, you will need to give something up; namely, you’ll have to trade him for it.
The Moblin will trade a surprising number of things for the Boomerang. In fact, the only items it won’t take are your shield, the Power Bracelet, Bombs, Magic Powder, or the Bow. Anything else is fair game!
… However, the best thing to exchange for it is the Shovel, especially if you’ve already managed to collect 20 Seashells and upgraded your sword. Once you’ve collected the required number of Seashells, the Shovel more or less becomes a worthless item. Well, it still serves a purpose, but it’s primary purpose is to hunt Seashells. Still, a Boomerang for a Shovel is a more than fair trade to boot.