Hi! These items are known to be unsellable, due to being quest-related, though some are not actually required to complete a quest. A full (?) list of such items is here.

If you’re short on gold there are other ways to get it (spoiler alert):

You can, if you’re not adverse to it, make all the gold you need by crafting items and selling them.

If you’d like another hint:
Make arrows.

If you’d like a bigger hint:
Charm arrows (in Act 2).