Selling a stack of potatoes is pretty good way to earn money.(512 coins)
If you just started, potatoes are definitely good to sell.
If you're further then potatoes: wheat, lapis, cacao beans, jungle logs, cactus, birch logs, melon, acacia logs.
All these are needed for quests so don't sell it straight away and kinda wait until it's over. You can look at my thread to know which are quests are next.