In any given market, some individuals will become buyers, and others sellers. If two goods are bartered, as in our cows-versus-hens market, the buyers of the first good are the sellers of the second, and vice versa. In any free market, the same individual may move from one capacity to the other as market conditions change. For instance, we saw that Smith would be a cow buyer if the market price fell to 11 hens, but a cow seller at a price of 19 hens or higher (pp. 4.6:2-9). The determining factor is the marginal value of the good to the individual.

So far in our analysis, we have assumed that each participant in the market sought goods only for immediate personal use. In the marketplace, a good may also have utility arising from its exchange-value, as in the next example.