These ideas definitely need to be thought through and balanced. But a limiting factor would be that the bigger the deal, the greater the risk. The smaller offers would probably be not too risky, but trying to trade on a restricted 1,000 bushel deal should be very risky.
Also, many of these options I posted don't really defraud anyone. Trading on a restricted deal means that the lord/steward posting the orders still gets the food or gold he wants. It's just getting bought/sold by someone other than intended. The options to actually defraud the posters and pocket an illegal profit would be even more risk.
The option for a banker to monitor trades for fraud is interesting. Perhaps it could be a police action style thing, limited to the physical location of the banker, or just his current and surrounding regions or something. I wouldn't want it to be just any three random offers from anywhere in the realm. Realms could possibly have dozens and dozens of orders. Monitoring three would be essentially useless. Rather, monitor a marketplace, and anything going through there would be scrutinized.
Wouldn't that block off too many offers? I think this could work if traders have a way to find out where the banker is. Then they can play cat and mouse.
