Research in the sociology of markets finds that shared meanings facilitate valuation and exchange by providing frameworks for perceiving and evaluating products and producers. Whereas studies of local sensemaking explain how meanings emerge in market interaction, and macro sociological accounts explain how meanings embodied in conventions, structures, and institutions are used in markets, understanding of the links between these two levels of analysis remains underdeveloped. In this paper, we propose a theory of how engagement and influence at the micro level gives rise to conventional labels and categories. Our theory proposes three processes through which audiences in markets come to share meanings: (i) through interaction among the audience; (ii) through influence of vanguard audience members on lay audiences; and (iii) through vanguard influence on authorities. We investigate some of the propositions on label use and category differentiation in 23 product categories on eBay.