I would like the ability to post a Purchase Order.
i would post that I want 50 onions and I'm willing to pay 200 currants and someone could browse the purchase orders and fulfill any quantity of the 50.
Another could post that they want 500 general vapour for 3000 currants, and sellers could fulfill it over a few days.
Having both buyer and seller postings would make the market a lot more accurate and sane, much like how stock markets work in the real world. Spiral Knights already has this for their Energy market, but I think they have not yet extended it to the commodities markets.