Sadly, buildings and animals cannot be sold or moved into storage. On the other hand, storing a decorative item is very simple. Tap on the decorative item, and keep holding until you see the arrow appear. You will see two buttons: one with circular arrows (used to rotate items), and a second one with a box. Tap on the one with the image of a box on it. The item will be stored in the store, and you will be able to retrieve it at any time.