The cited Wikipedia article on trade printing describes the mechanism in full (wholesale sale of printing materials and services to brokers, designers and agencies who resell to the end client under their own name) but gives no year, decade or historical event for when this arrangement began; a companion Print broker article that might have carried the missing history does not exist on Wikipedia (404).
What would resolve this A citable printing-industry history source naming when wholesale, unbranded printing for resellers first emerged as a distinct business model.
Openbusiness historyWikipedia, Trade printing