Laborers are free to work where they want, if qualified. Consumer is King and ultimately decides what the capitalistic economy should produce, and the resource providers must make their free choices within these constraints. When many buyers and sellers are in a particular market, no one buyer or seller will be able noticeably influence its price. The idea of pure competition is that there are so many independently acting sellers that each, because each is contributing an almost negligible fraction of the total supply, has virtually no influence over the supply or, therefor, the price - No Rigged Markets. The same reasoning applies to the demand side of the market.
Buyers are plentiful and act independently.
The widespread diffusion of economic power underlying competition controls the use and limits the potential abuse of that power. Competition implies it is simple for producers to enter or leave an industry. Capitalism is a market economy. The preferences of sellers and buyers are registered on the supply and demand sides of various markets, and the outcome of these choices is a system of the product and resource prices. These prices are guideposts on which resource owners, firms, and consumers make and revise their free choices in the furthering their self-interests.