I visited all the vendors I could get to to double check (just the 2 at the inn). There was one outside the crypt who wanted an egg but it's not letting me leave the zone. I don't recall selling it but it is possible I did it by accident I guess as it just shows as a standard white item apparently.It's tricky because whilst there are several broken Moon Lanterns in the game from various sources that you can repair, I think you need to get to Moonrise Towers and Balthazar's chambers in order to do so. Other magical light sources might be able to get you there. If the game is pushing you in that direction anyway give it a try.
Also, are you 100% sure the pixie bell item isn't on a vendor somewhere, or misplaced in a bag? Everything in this game is persistent. Vendors hold onto items you sell them indefinitely.