Markets are physical or virtual places in which individuals and firms buy and sell products and services, sometimes competing directly and always indirectly. (Even a so-called monopolist must compete for the consumer’s dollar.) A market transaction occurs when a buyer gives a seller something of value in exchange for a product or service.
Some commenters have suggested that there’s no marketplace of ideas. They’re right, insofar as there’s no exchange taking place — ideas for money or something else of value. But there are competitions among ideas. Those competitions involve active vendors of ideas (e.g., religious, political, scientific), who vie for adherents, even though the vendors may receive no payment from their adherents.
Unlike true markets, where competition usually eliminates sellers whose products and services are found wanting, the competition of ideas often leads to the broad acceptance of superstitions, crackpot notions, and plausible but mistaken theories. These often find their way into government policy, where they are imposed on citizens and taxpayers for the psychic benefit of politicians and bureaucrats and the monetary benefit of their cronies.
The “marketplace” of ideas is replete with vendors who are crackpots, charlatans, and petty tyrants. They run rampant in the media, academia, and government.