The broker in an industrial ecosystem
A broker may take responsiblility for maintaining the cohesion of a broader exchange network as well as for sales and marketing. The by-product exchange broker would have economic self-interest in discovering new opportunities for bilateral exchanges and facilitating their occurrence.
The broker would help companies perceive their common opportunities and explore opportunities for increasing effectiveness of the whole web of exchanges (although the deals would likely remain bilateral).
There is a potential downside to depending on brokers. Since they usually work on commission, they have no incentive to seek reuse of lower value materials. They tend to focus on higher value by-products.