Currently buyback function is disabled because there is(was) a bug that enabled players to re-sell/re-trade items they got from other players and/or market.
Would it be possible to treat all non-equipment items that you bought back from NPC as received from other players/market marking them as not tradeable anymore in the process?
That would fix the exploit with buyback feature, players get to enjoy their ability to revert some of their “Oh shi~!” moments and it offers a moderate punishment for not paying attention.
Win/win/win situation imo.