Minecraft wiki entry for Things_not_to_do, associated text: Some villagers want hard-to-obtain materials, and give you a low number of emeralds. Some of the cheapest trades for obtaining emerald are trading sticks to fletchers, trading crops to farmers, giving rotten flesh to clerics, etc. You should only sell hard-to-obtain materials if you are trying to level up a villager's skill level and you already have a lot of that resource.