For example, buyers and sellers are two common types of agents in partial equilibrium models of a single market.
This principle was established by the United States Supreme Court in Reeves, Inc. v. Stake, 447 U.S. 429 (1980), in which the Court upheld South Dakota's right to give South Dakota residents preferential treatment in the purchase of cement produced at a cement plant owned and operated by the state.
"Nothing in the purposes animating the Commerce Clause prohibits a State, in the absence of congressional action, from participating in the market and exercising the right to favor its own citizens over others."
In Reeves, 447 U.S. 429 (1980), the Court relied upon "the long recognized right of trader or manufacturer, engaged in an entirely private business, freely to exercise his own independent discretion as to parties with whom he will deal."
[citation needed] By contrast, discriminatory practices in the provision of essential public services, such as welfare, police protection, and primary education,[citation needed] would violate the Privileges and Immunities Clause, and discriminatory practices when the state is acting as regulator rather than market participant would violate the dormant commerce clause.