I seem to remember reading that you might need to keep crystals, but the rest you can sell. I store all items, that I think I might need eventually, in the secret stump just north of Hillbrandt. I've got troll bile, crystals, parts of a dagger, etc, all just in case. I also keep scrolls of spells I can't learn yet. If I wouldn't restart games so often, I'd be done with Inquisitor. Restartitis is an incurable disease for me.