"Stealing" takes place during the investigator trading phase; it's essentially just mandatory trading.Sure? The german rules state that only the following cards can be traded: "Keys", "Equipment", "Weapons", "Allies" and "Books". Nothing else. Not even "Artifacts".
Equipment, Tomes, Allies, Artifacts, Keys, Weapons, and Story Items can all be stolen, even if they are Starting Items. Clues and Spells cannot be stolen, as they are not tradeable.