NPC's are mobs, mobs are not enabled in the main world. Thus no NPC trading will happen in the main world.

As well NPC trading wont happen in say the Nether or Mining where mobs are enabled because you can simply trade around and earn cheap diamonds or other items and will trash the economy.

I can trade 16 wheat for 1 emerald and then trade 7 emeralds for a diamond chestplate. So your diamond chestplate is now only worth 112 wheat?

Villagers charge a much higher price at the start but prices drop dramatically the more you trade with them and with so many users everyone would be getting the super trade deals within a day or two.

I highly doubt it will ever be added unless there is a way to totally control the prices but then it becomes a trade station or equivalent exchange and still kills the market for people selling items. Why auction/sell my items if I can trade A for B and then B for C and get what I want instantly.

RagMan

 

Andrewkm already said that this will never happen, sadly