Yes to buyback.

However, this raises another concern of mine regarding the NPC interaction. Instead of choosing from seperate "Buy" and "Sell" options after talking to a NPC merchant, these should be included in the same choice along with the "Buyback".

NPC merchandise to purchase would be far left tab, followed with your inventory to sell from, and then the Buyback tab.

[NPC Inventory] [Your Inventory] [Buyback Tab]

Also, eliminate the "Are you sure" window popping up every time you sell something. Just check the buyback list to make sure you didn't accidentally sell anything. Sold items are removed from the Buyback tab once you exit interaction with that NPC.