Bukkit

I am trying to write a sign that let's me sell enchanted books, in my exact scenario mending books I bought from a villager. However it keeps saying that "You have specificed invalid item id!". I have tried many different names and number IDs for the mending book, including 403:2 403#2 and EnchantedBook#2, etc. with no luck. Does anybody have any ideas of how to fix this?

Please use the exact name provided by /iteminfo or simply use the auto fill functionality by writing a question mark on the last line.

If it still doesn't work then please make sure that you are using the correct version for your server version (latest development build for anything above 1.13) and that there are no errors on startup or shop creation in your console/log.