According to the Thief's character card, he can steal ANY object from the Purchase deck, even if the item is unavailable to buy.
Another issue I had was trying to cast Immobility on the Sentinel at the start of a battle, but I could only queue the spell and could not cast it until the next player's turn, where I could cast it on any character or creature on the board. I cast the spell on the Sentinel, but when it got to my turn, the spell's effect wore off.