I've always though the bankers should receive notice of all large caravan purchases, even if a trader is not involved.
Yes but anyway consider this situation: harvest day, at sunrise regions gather food, threshold for automatic transfer is surpassed, all regions send caravans that are invisible on the way, the banker has absolutely no clue, if not going and checking how much each regions harvested, of how much food is around. He has no clue either whether the food is going here or there, in the realm or out the realm, and he would be informed only as caravans sell food to the cities, that is likely to happen in waves according to the distance.
So rather than being clearly informed on the food status of his realm he has to half guess half calculate what's the situation, asking lords where did they send the caravans and believing them on the word (since he can't check). In short: a nightmare for planning.
