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.
Best Answer
I've had several items stay far, far, longer than a day in a vendor's shop before. I am not sure if they stay indefinitely, though they seem to take a lot of time to go away.
The UESP wiki lists two distinct types of permanent items:
Type 1 - Permanent (fully)
Type 2 - Permanent (until dropped)
List of permanent items
The following items were found to be fully permanent:
The following items were found to be permanent until dropped (or placed on a weapon plaque):
Note: Permanent weapons may fail to prevent corpses from disappearing, although they will remain in respawning containers.