Time manipulation and money
Locate the Traveler's Ship (the guy who gives you all of the treasure). Kill all of the enemies on board his ship, and then talk to him. After receiving the treasure, exit his ship and save your game. Turn off your DS, turn it back on, and head into the DS settings. Adjust the time and date (moving it forward one day, for instance), and restart your game. Return to the Traveler's Ship, repeat the same process as before, and you'll receive more treasure. Stock up using this method, and then go sell all of your goods. You can get 9,999 Rupees easily in about twenty minutes doing this.