Detailed Idea Description
Now, if you wanted to trade, for example, ONLY mending books, at best you could trade all enchanted books with sign shops.
I would suggest making (if it's possible, of course) that if you click on a sign with a mending book, it would only trade mending books, not all enchanted books.
The same would be quite nice with heads. For instance, it could trade only heads of a specific nickname.
It would also be very good with keys, so that you could trade vote, gold, diamond, emerald, event, and vip keys separately :)