At the moment, we've marked some items as unsellable if they meet the following requirements:

-If the items is no longer purchasable in the shop
-If the building or decoration drops items
-If the building is associated with a character's action.

We've made these decisions to avoid irreversible mistakes. We don't want to risk players losing an item they cannot purchase again, an item that drops necessary items, or a building that is currently being used by a character.