So in the Trade Cube and Outside (when the librarians are running around), I can't buy Enchanted Books. Either it is not displayed or the bookshelf is selected as the purchase.

Also, it has problems with the mod "Carryon". I can carry items around with the mod and could not get the reading desk out of the trade cube while the mod was on.

I have made a creative world and have made a small video above the promlem.

Other mods
at the end of the video is a complete list of all active mods (without carry on)

Link too the Viode of the Bud (ist too big)
https://youtu.be/Rl7eysLBswI

I think you don't understand villager trades in general.
You need to provide the emeralds and a book to buy an enchanted book.