Stop merchants from autoequipping items sold to them

Making a separate pull request because I'm not 100% sure this won't cause any side effects.

Items added via console (additem) will still be equipped since the add-by-id function sets the owner of the item to the NPC it's added to.
Items sold to an NPC currently do not get the owner set to the NPC they are sold to, meaning they won't be equipped.
Afaik we do not need to set the owner in that case. It would only really be relevant when stealing back items you previously sold to the NPC, then being able to sell them back. If that happens, we could set the owner upon taking an item from the pickpocket window?