i know its in a secret room in the crypt. already found it but do not have it in my possession. if i sold it to a dealer can i buy it back

You could try having someone else pick up the quest for an item respawn. Also, some enemies have a small chance to pickpocket an item (including quest items). I usually keep all quest items on one person and check their inventory after fighting specific monsters. Nymph, Ratling, Bandit, and Imp-type enemies can steal them and then drop them on death. The easiest way to locate a pickpocketed quest item is to leave the area (to despawn unimportant items), then run through with Spirit Eye. I didn't yet test GOG's version of the game with quest items on merchants.

thanks