A lot of trades have balance issues when accounting for Zombie transformation and the Hero of the Village effect; buying bookcases for one emerald, then selling its books for more emeralds, then using a single one of those emeralds for what was supposed to be an expensive enchanted book. Stoinks! An hour got me a chest full of Mending enchants, this should really be patched soon.

Trade exploits seem to have been patched, I can't replicate in newer versions.