In theory, there is this merchant at the north side of the Novigrad island who is supposed to buy your things at the price that appears in the inventory. Yet for some reason it appears as a normal merchant who buys stuff at a lower price. Why does this happen? Am I supposed to do something for this merchant to work?
This king of behavior is usually considered as an exploit and not intended to be in the game. Can't find an exact patch which fixed that, but it was fixed as the famous cow and sea shell bugs too. I just checked in-game - the merchant is there and buying items with lower than value prices, selling just empty bottles.