… is from page 55 of Armen Alchian ’s and William Allen’s Universal Economics  (2018; Jerry Jordan, ed.); this volume is an updated version of Alchian’s and Allen’s magnificent earlier textbook, University Economics:
It is often argued that intermediaries like wholesalers, retailers, sales agents, and advertisers exploit the ignorance of customers. That is true in the same way a teacher exploits the ignorance of students, doctors exploit the ignorance of patients, and authors the ignorance of readers. Intermediaries exist (survive) because they reduce the costs of exchange.