Some items in Highrise are untradeable. If you have duplicate untradeable items or spares you don't want to bother selling through the storefront, you can still purge them from your inventory!
Pawn untradeable items in the Welcome Plaza. This room is always at the top of the rooms directory.
There's a bot standing in the back of the room with a "Sell" icon above his head; tap on him to receive Bubbles for items you no longer want.