I think you are out of luck.
Vendors doesn't seem to loose items so if you can check them all out you might be able to find buy it back again.
Fortunately I believe that the quest is not mandatory. Just accept it and continue on with the game.
Funny, this as if this happened in real life. We forget how much games usually hold our hands when it comes to this kind of thing.
(I lost my Golden Grail once and spent an hour until I found the girl I sold it to)
