This answer in the question, "Can you rebuy things from fences if they don't sell that item in skyrim?" states that:
Unique items (e.g. Dragon Claws) remain in the inventory of the storekeepers indefinitely. However items such as armour, whilst they will remain for a day or so, are liable to be lost permanently when the storekeeper's merchandise chest resets.
Aside from Dragon Claws, are there other "unique" items which will stay in a merchant's inventory forever? What determines if an item is considered "unique"?
I would like to know as I regularly sell items I don't need to merchants, so they can be "deleted" in the game when the merchants have their inventory reset. I am not sure which items will indeed be removed with a merchant inventory reset.