+1 for material contracts.
We've talked about the hauling aspect in other threads, and there's a lot of aux. stuff that would have to be in place in order for hauling to work. Limiting what the hauler can pickup, what they can do with it when its in their possesion, and how they deliver it and get paid.
First, the load would have to be 'encrypted' making it useless to the hauler; this works because bots don't move real items anyway, basically its an item that only the shippee can unpack. Then an automatic way to get paid once it is delivered.
Anything short of that, and it may as well just be a bulletin board with requests and then you PM the contact. Or just use the market.