A Merchant's Oath is a binding verbal contract between two merchants, formed by trading one product for another. Link is first introduced to the concept by Zunari, the proprietor of a humble stall on Windfall Island, who asks him to negotiate supply contracts on his behalf with Traveling Merchants. Once Link trades an item with a merchant, forming such an oath, the merchant will thereafter ship their products to Zunari's shop, increasing his product line. He gives him a Town Flower to trade for another item, which begins the Island Trading Sequence.
Whenever Link encounters a Traveling Merchant while on the Great Sea, he can form a Merchant's Oath with him and receive a variety of items, depending on what he has to trade. The Traveling Merchants honor their oaths by sending their items to Zunari's shop via Postman.
- "So, by trading your goods for my goods, we've formed a merchant's oath." — Traveling Merchant (The Wind Waker)
- "I will sell anything! I am a master of the sale! I just need to get some new products! Could you not seek out traveling merchants and negotiate supply contracts with them for me?" — Zunari (The Wind Waker)
- "No, no, no! Do not look so frightened! I say contracts, but it is just a word. You do not have to do anything very difficult. All I need for you to do is trade one of my products for a new product that the merchant has in stock. You see, among merchants, a trade is proof of a contract! That is to say, it is the merchant's oath... That is what we call it. Yes, yes." — Zunari (The Wind Waker)
- "Merchants that have sworn a merchant's oath will afterwards send their products to me. It is the way things work. So, as you succeed in trading my products, my product lineup will increase!" — Zunari (The Wind Waker)
- "I give to you my proof of contract... In other words, this is my merchant's oath! Take it with you, my young partner! The first thing you need to do is take this to another traveling merchant and trade it for something...to build on the foundation of my merchant's oath. As soon as you trade, the merchant you traded with will send his products to my shop, thereby increasing my product line. Which will make me quite happy!" — Zunari (The Wind Waker)
- "Thanks! That settles things. I'll give you your item, as promised. Hold out both of your hands... So, by trading your goods for my goods, we have formed a merchant's oath. So, from now on I shall honor our merchant's oath by sending shipments of my product to the Windfall Island Shop." — Traveling Merchant (The Wind Waker)
- "How, you ask? Why, my good friend the postman will take it there for me, so it will probably get there before you do. Just go back to Windfall and check for yourself." — Traveling Merchant (The Wind Waker)
- "Yes, yes! Young master, look! Already, my shop has a wide variety of products to choose from! It is a cornucopia! Thanks to you, young master, every morning the postman comes with packages from the traveling salesmen you traded with." — Zunari (The Wind Waker)