Pineapple seeds are a popular item to trade on the Grand Exchange, because of their ever-varying price range. They can be obtained by looting Magpie implings, Bird's nests, from the Vinesweeper minigame for 10,000 points, or as a fairly common reward (1 seed) from Treasure Hunter, where it could be converted to 1000 coins during the Double Cash period, implying that normally they could be converted to 500 coins, which is less than selling them on the Grand Exchange.