I'm often disturbed by how few lords know how any of the economy works. I have to train people all the time on how to even send a manual caravan all the time, or that having their tax rate low might be nice on the peasants but it might also explain why they're asking for money to keep a unit in the field. Mostly just have to be patient.
I've been at this game for a very long time (and have spent a lot of effort to understand the economic and good system of BM) but the buy/sell stuff is still a bit confusing when it comes to stewards. It just doesn't come up enough for me to learn about it in detail. The explanations here so far have cleared up a few things, but I'm not sure the current system is better than the one I proposed so far. I do like that if I'm ever a steward again, I can really tank my region if I want.
The current system is already implemented. Yours requires a coding change from the Devs who are already overworked and have a massive todo list. Unless the change offers a large improvement the chances of it happening any time soon are very low I would think.
