A market is one of the many varieties of systems, institutions, procedures, social relations and infrastructures whereby parties engage in exchange. While parties may exchange goods and services by barter, most markets rely on sellers offering their goods or services (including labor) in exchange for money from buyers. It can be said that a market is the process by which the prices of goods and services are established.
For a market to be competitive, there must be more than a single buyer or seller. It has been suggested that two people may trade, but it takes at least three persons to have a market, so that there is competition in at least one of its two sides.
However, competitive markets, as understood in formal economic theory, rely on much larger numbers of both buyers and sellers. A market with a single seller and multiple buyers is a monopoly. A market with a single buyer and multiple sellers is a monopsony. These are the extremes of imperfect competition.