It doesn't feel like the merchant who sells the tomes in the consortium is able to be bartered with. I have offered MUCH better trades than should be denied and have had no success. If its not a bug, seems rather unfair to those that don't use/or have a hard time getting gold.

Certain shopkeepers are only interested in certain kinds of goods and while you may be able to barter other things with them, the markdown can be very steep. When you list this particular shopkeeper's goods you'll see that he likes scrolls, so offering him a scroll is going to yield the best deal you're likely to get.