The effects on the market price of a change in the number of buyers, in the number of sellers, in the prices sellers set and in the buyers’ reservation prices are established in a simple network model of a market.

Two results showing the limitations of the “as if” methodology are proved under relatively mild assumptions. In an interpretation of the results, a competitive market cannot simulate the outcome of a market M in which the single price assumption does not hold. In a second interpretation, the...