To sell an object, tap the icon with blue arrows in the bottom right corner of the screen, select Sell (the icon with coins) and tap the object you want to sell.

Tap the red cross in the bottom right corner of the screen to exit Sell mode.

If you want to sell some Harvest from Storage, you’ll need to go to Big Land or Love Island, then to Storage >> Harvest tab and tap Sell button under the item you want to sell.