The obligation to use certified weights and measures is a recurrent topos in Greek literary and epigraphic sources relating to market regulations. Magistrates were required to provide merchants with measuring instruments and weights and to control the use of official weights and measures in transactions. These controls took the form of countermarks or other types of marks on the instruments and gave legal status to them. These mechanisms were put in place notably to prevent fraud and to preserve confidence in the economic system. In this paper, we propose to study the customs and regulations surrounding the daily use of weights and measures in the market and the role of public authorities in the control of measuring and exchange instruments. Our aim is to shed new light on the modalities of economic transactions as well as on the practices and representations of weighing operations in the ancient Greek world.