When two people not under duress enter into an exchange for goods or labor services, both must be expecting to benefit or the exchange would not occur. In any such exchange there necessarily exists a double inequality of value. Each trader gives up something to obtain what he or she prefers. Moreover, we have at least prima facie grounds for pronouncing the exchange legitimate since no compulsion is apparent.
But as Sheldon Richman observes, libertarians must take care in applying the free-exchange principle. Let's say you enter a post office and buy a first-class stamp for 45 cents. May we conclude that you prefer the services of the post office, a government-protected monopoly, to whatever else you might have spent the 45 cents on? Weren't your alternatives artificially constricted by a system supported by violence?