"Selling Free Software" addresses one of the most persistent misunderstandings about the free software movement: the conflation of "free" in "free software" with "at no charge." stallman argues clearly that selling copies of free software is entirely legitimate and consistent with the free-software-definition, provided that the four-freedoms are respected — that buyers receive the source code and the rights to study, modify, and redistribute.
The libre-vs-gratis distinction is the conceptual center of the essay. "Free software" means free as in freedom, not free as in price. The gpl-v2 (and its predecessors and successors) explicitly permits selling software: it requires only that anyone who distributes a GPL-covered program also makes the source available and passes on the same freedoms. A company can charge whatever the market will bear for a distribution of free software; what it cannot do is restrict the recipient's freedoms.
stallman also addresses the concern, sometimes raised by commercial developers, that allowing redistribution undermines the value of selling software. He argues that the business model enabled by free software — service, support, customization, training — is more durable than one based on restricting copies, and that the free software ecosystem as a whole benefits from having well-funded developers contributing to it.
The essay has practical implications for understanding the gnu-project's own funding model and the free-software-foundation's practice of distributing GNU software on physical media for a fee. It is a corrective to both audiences: to potential contributors who think free software means unpaid work, and to potential users who think free software must always be obtained at no cost.
The argument here also addresses a recurring criticism from the open source community, whose members sometimes suggested that the insistence on "freedom" language was confusing the market. stallman holds that the confusion is worth enduring because the alternative — "open source" language — loses the ethical argument entirely, a position elaborated in why-open-source-misses-the-point.