As a lord buying food, 10% wouldn't even make me blink. I'd still do it without hesitation. At the rate food rots, at least a third, if not more, of it will rot before I ever get to use it. I would much rather have the food right now, and know I have it, than post an offer and hope someone will fill it. Like they say: a bushel in the warehouse is worth two in the market.
I don't think this penalty will do anything at all.
IMO, what would help traders the most, is an option to partial fill orders somehow. The need to match orders exactly really stifles things.
My problem with traders right now is that there are
no orders to fill. Not even large, unbalanced orders. There's just a bunch of buy orders and an occasional realm only sell order.