The characteristic of capitalism that buyers and sellers will freely and willingly exchange in market transaction is called

Voluntary exchange.

The basic principles of capitalism were laid out by Scottish philosopher Adam Smith in an influential book published in 1776:  An Inquiry into the Nature and Causes of the Wealth of Nations.  The Nobel-prize winning 20th century economist Milton Friedman said of Adam Smith, "“The key insight of Adam Smith's Wealth of Nations is misleadingly simple: If an exchange between two parties is voluntary, it will not take place unless both believe they will benefit from it."  Smith argued for such voluntary exchanges within a free market system.  He labeled the government-manipulated economic system that was prevailing in his day as "mercantilism," a system in which governments specifically authorized some merchants as the official agents of commerce (rather than endorsing free enterprise).  The mercantilist system also viewed wealth as though there were a fixed amount of it available in the world, represented by precious metals such as gold and silver, and that nations were in competition over who got more of that fixed amount of world wealth.  Smith saw that wealth was something that could be created and increased through voluntary exchange and free trade.  Smith's ideas formed the basis for what we have come to know as capitalism.