I have a number of duplicates. I don't recall if I was ever able to find a buyer for them last time I played. I am playing UU1 right now. Anyone know?

That...has never occurred to me to try. I genuinely don't know whether they have any trade value. All rune stones I find go right into the rune bag, regardless of whether they are duplicates. I don't think it actually does anything except get the rune stone out of inventory without leaving it lying around somewhere.

If I need barter goods, I just go fishing.