If a thief is already wearing a hat in Munchkin, is it possible that he can attempt to steal someone else's hat, even if he can't wear it? What about if he sells it immediately?
From the rules:
All Items you have in play are considered "carried." Items that are actually giving you a bonus are "equipped."
Anyone can carry any Item (except for extra Big items), but you may equip only one Headgear, [etc.] ... If you are carrying two Headgear cards, for instance, you can equip only one of them at a time.
This means that even if an item can't be equipped (because you already have an item equipped in that slot, or any other reason), it can still be carried, so a thief can still steal it and just carry it.
I found this on the munchkin forum.
When the Item is stolen, it stays in play.
I guess this means in order to steal the item you must be able to keep it in play. If you have the open space, you can steal and sell immediately, but if you cannot keep it in play, you cannot steal.