Finding TreasureThe random treasure pieces that Link will find are quite easy to come by. Here are several of the things Link can do to get treasure.
1) Purchase treasure from the various island shops.
2) Use the salvage arm to bring up sunken treasures from the ocean ground.
3) Some of the various mini-games will award treasure for getting high scores.
4) Find them in various treasure chests all throughout the overworld and temples.
5) Trade treasure with various people via Nintendo Wi-Fi.
As Link goes through his journey, he will surely collect a ton of random treasure pieces. They are commonly sold and replenished within the shops on Mercay, Molida, and Goron Islands. In addition to that, there are two sunken treasure locations that offer a treasure piece as a reward. You can view those pages by clicking here. The various mini-games that Link plays all award Treasure as well. Usually treasure is the worst of the major rewards, just under ship parts and larger rewards.
Many of the islands will have treasure chests lying around that contain treasure, as do the temples. In particular the Temple of the Ocean King has unlimited treasure for Link to obtain. After Link has acquired the Phantom Sword late in the game, head to the Temple of the Ocean King and begin heading down the many floors. Defeating all the phantoms on floors B1, B2, B3, B4, and B6 will created treasure chests that all hold random treasure pieces. Link can obtain these treasures, leave the Temple, and then re-enter it. All the treasure chests will be reset so Link can fight off the phantoms again and gain more treasure.
Trading TreasureThe game allows players to use Nintendo DS Wi-Fi capability to trade ship parts and treasure with friends all around the world. What you need to do in order to begin the trading process, is to meet up with Freedle, the instrument playing dude at the top-right of Mercay Island. If you don't know how to reach him, refer to section 8.1 within chapter 8 of the Phantom Hourglass Walkthrough.
There are two methods that you can actually trade treasure online. Tag Mode is the simplest way to go if your friend is nearby. Either place a treasure in one of the three boxes near freedle while you are in game, or in tag mode. Then go to tag mode and if there is somebody nearby also in tag mode, the treasure will be traded.
Battle Mode is the other option of trading treasure. Once again, place the treasure in one of the boxes near freedle. After you setup and complete a multiplayer match with an online friend, the treasure will be traded. The game has to end and if one player quits out early there will be no trade. The outcome of the match is irrelevant, as the parts will be traded regardless.
Treasure PricingOther than trading treasure, Link can also sell it to the Treasure Teller on Mercay Island. The treasure prices are all randomly determined. They will either sell for 50, 150, 800, or 1500 rupees. Each game is complete different, so a Pink Coral in one game might be worth 1500, but in another it might only sell for 50. Thus, trading for specific types of treasure is an easy way for Link to make some easy money. There is one exception to this and that is the Regal Ring. Regardless of what game you are playing, the regal ring will always sell for 1500 rupees at the treasure teller's hut.
The Ho Hos on the traveler's ship in the southeast quadrant will also purchase various treasure from you. Which treasure they purchase varies from day to day. They will offer slightly more than what the going price is at the treasure teller's hut.
Below is a complete listing of all eight of the random treasures that Link will find on his quest with names, descriptions, and images of the treasures.