Interacts with an item in a slot offered for trade by the target. Only meaningful when used with the last (7th) trade slot: if an enchantment-type spell is currently awaiting a target (i.e. the glowing hand cursor is showing), targets the item in the given trade slot for the enchantment. (The enchantment to be applied then shows for both parties in the trade, but is not actually performed until both parties accept the trade.)