The Boomerang Dealer is a character from The Legend of Zelda: Link's Awakening. He is a Goriya that lives in a cave on Toronbo Shores. He can only be seen with the use of the Magnifying Lens, and thus, Link cannot speak to him until he completes the Quest for the Magnifying Lens. Once he does, however, he will offer to trade Link the Boomerang for whatever item is set to the B button; however he will not accept a shield, Bombs, Power Bracelet, Magic Powder, or the Bow.
The Boomerang is extremely powerful, and after the trade has been done, the Boomerang Dealer will plead for Link to trade it back, after which the process of trading items for the boomerang can be repeated.