The Marxist concept of value is radically different from the Ricardian con-cept of value as ‘labor expended’. Unlike the Ricardian theory of value, the Marxist theory of value is a monetary theory. In the Marxist system, the value of a commodity cannot be defined in isolation, but exclusively in relation to all other commodities, in the process of exchange. In this relation of exchange, value is materialized in money. The essential feature of the ‘market economy’ (of capitalism) is thus not simply commodity exchange but monetary circulation and money. Barter is for Marx non-existing, as all exchange transactions are made up of separate acts of exchange of commodities with money.
read here