The right to trade with other individuals in the absence of physical force, compulsion, or coercion is based on the concept of individual rights, specifically "the right to life".

The "right to life" is derived from the law of identity: X is X. In other words, a living being is a living being. Living beings exist only as long as they remain alive, or have life. As such, living beings must constantly sustain their own life through self-generated action.

A "right" is a moral principle defining and sanctioning an individual's freedom of action in a social context.  Social interaction exists only through living beings.  As such, social interaction is possible if and only if living beings have the freedom to engage in self-sustained, self-generated action.  This right, the freedom to engage in self-sustained, self-generated action, exists only for individuals in a social context--and is what I term "the right to life."

Thus, "the right to life" is the most fundamental right of an individual.  In a social context, freedom of action is violated only by means of physical compulsion, coercion or interference by other men.  So, for every individual, "the right to life" specifies the freedom to think and act, to pursue one's own ends through voluntary, uncoerced action.  To pursue the property which enables one to sustain their life, and the happiness which makes life worthwhile.  Consequently, an individual's "right to life" imposes no restrictions or obligations on the actions of other individuals, only to abstain from violating their "right to life."

Many corollaries stem from an individual's right to life, one of which is "the right to property."  Living beings, in a social context, possess the freedom to sustain their own life.  Life is sustained, through self-generated action, by obtaining and consuming objects such as food and water, or by producing and utilizing objects such as clothing and shelter. The freedom of action to consume, utilize, or dispense of objects requires ownership.  Property consists of the objects under one's ownership.  Thus, individuals with "the right to life" must also have the freedom of action to consume, utilize, or dispense of objects under their possession--that is, they must have "the right to property."

A free market is the consequence of individuals exercising their "right to property" by voluntarily exchanging (dispensing of) objects under their possession.  Individuals are motivated to engage in voluntary trade and cooperation because each believes they gain the ability to better sustain their life in comparison to the alternative.  This simple fact assures that trade will occur if and only if all parties perceive a benefit from the terms of exchange.  For instance, if a baker desires meat to supplement his bread and a butcher desires bread to supplement his meat, each is free to exchange one for the other, or to use an intermediate currency.  The baker likely has neither the time nor money to butcher meat, so he benefits from baking a little extra bread to obtain the meat he desires, and vice versa.

In the absence of third party interference, individuals are free to abstain from trade just as they are free to engage in it.  Thus, trade will persist only as long as both parties perceive a benefit.   If the butcher demands too much in exchange for his meat, or the baker too much in exchange for his bread--neither has any recourse to force the exchange.  Each will suffer from a reduced availability of goods until the terms once again become mutually beneficial.

In conclusion, the philosophical basis for a free market is individual rights--not any perceived economic benefit.  Any economic benefits which result from the restriction of physical force or coercion in the trade between individuals should be regarded as secondary.