A Villager that buys and sells more Mob Drops

Mob Drops - especially in the later stages of the game - become quite unimportant, which leaves Mob Grinders or AFK Farms useless. An NPC that would buy Mob Drops and Sell them as well would help with that!

Here's an example:

32 Gunpowder - (you get) 1 Emerald
64 Bones - (you get) 1 Emerald
(you give) 2 Emeralds - (you receive) 4 Spider Eyes
(...)