Assuming this also affects other versions. Seems to have affected past versions as well, but I don't know which ones.

The trading feature does not work with addons (i.e. you can't make a mob capable of trading in an addon). Even if you do an exact copy/paste of the trading code from villagers, no other mobs can be made to trade. (I have seen addons that boast of allowing other mobs to trade but I have yet to see one that works).