In most markets trade does not involve just producers and consumers but also one or more
middlemen serving as intermediaries. For example, brokers and market makers fill this
role in financial markets as do wholesalers and retailers in many manufacturing industries.
Classical economic approaches to studying markets, such as competitive equilibrium analysis,
largely abstract away the role of such middlemen; a point made in the introduction of
Rubinstein and Wolinsky (1987), who attribute this is to a lack of modeling how trade occurs
and the associated frictions involved