Maybe it is just the built in minecraft villager reputation system? Where once you kill enough of them they will stop trading with you?
Not that it matter anyway, we have the smeltery to get the emerald out of their system.
Thank you both for the quick reply - now you mention it I do remember something about villager trading in vanilla being affected. If that is all it is I will try, I might as well as I have already killed a dozen or so getting the magician's orb, what's a few thousand more!