If I remember correctly, bankers get informed of large trader-initiated transactions within their own realm. Thus, if a trader goes and buys 1000 bushels in Shoka or someplace, Terran's banker will hear about it...but won't know where that food gets sold. When the 1000 food gets sold in Paisly, D'Hara's banker will hear about it, but I doubt he'll be inclined to tell Terran's...
Also, I think that the 50 gold/100 bushel limit is a thing of the past. Not 100% sure, though; you might try it out.
