As far as I can see, if traders are brokering trades, it's because lords/stewards are lazy. I have yet to see a trade brokered in any of the regions I have a character in. This includes my lord characters on Dwilight and FEI, my steward on EC, and my knights on AT and BT. (All of these are city regions.) I have seen lords sell me food. I have bought food from other regions. But I don't think that any trader has ever brokered any of my trade deals.
It would be great if AT were working as designed. The question then would be why is it working on AT, and not the other islands? Is the food balance right on AT, and really wacky on other islands? Or is your character in Coria in just the right place to take advantage of it?
Personally, I think we really need more feedback from other people on AT. I have heard from at least one duke on AT that they are almost out of food, no one is selling them food, and there are no Sell offers available.
