Ransack the Mercahnt(Beetle's Ship)
Beetle's ship can only be accessed once a day; if you buy everything he has, he can only sell you potions. Beble's ship is a great place to get cheap treasure and rare ship parts (depending on the day). Go to his ship, and buy everything he has. Save the game, then turn off the DS. Turn it back on, and change the system date. Turn off the DS, then resume the game. You should start at your save point. Go to Beetle's ship, which should be closed. He should now have more or new stock.