So, I play two lord characters, both lords of cities. The way I work is to try and make sure I always have Buy orders open. That way people can always sell me food, if they want. And not just the cheesy 10/100 orders, but serious Buy offers. Then whenever I am traveling, or whenever I feel like it, I check the market and buy any orders that look good.
I decided to work out what would have to happen for me to decide to simply post offers, and let traders fill them for me. I can think of three things:
1) being unable to fill orders. I.e. lords can only post and hope a trader does it. I think this is unworkable, but list it for the sake of completeness.
2) Extortionate fees for non-traders. A simple 10-15% increase won't stop me. If you start getting to 50% or more, that might start making me think before I hit the button. But I will still do it if I think it is important.
3) greatly reduced availability of trades for lords. I can think of two things off-hand. First, restrict lords to trading with adjacent regions only. Second, make newly posted orders visible only to traders for the first three days after they are posted. Lords can only see them once they are 4 or more days old. This allows the traders to get an exclusive shot at all new offers before lords can act on them.
In addition, make more orders available to traders. Make it so traders can always see trade offers in their trader bonus range. (100 miles + skill based range). Then when they are actually at a marketplace, give them the extra 400 miles of range.
And finally, let traders partial fill orders. If it is that important to some lords that they not be partial filled, the let that be an option when posting.
But as things stand now, my trader on EC is feeling pretty useless. He's also a steward, and can't even find food to buy, let alone matching sets of orders to broker.