The medium of exchange function is redundant because the auction generates the real rates of exchange between all goods. Problems associated with real world barter, such as the double coincidence of wants, do not arise in a time-0 auction so the medium of exchange function of money is not required to solve them. In the real world money is an innovation that reduces risk and increases the choice, trade and production sets of agents. The Arrow- Debreu auction in a world of complete markets circumvents these challenges of the real world obviating the need for money. Goods buy goods in.
