In his book Das Kapital, Karl Marx puts forward a theory of capitalism as a social system. His conceptualisation of commodities is central to his argument (see for instance Marx 1867: chap. 1). For Marx, the capitalist mode of production is a system which produces and circulates commodities; that is, capitalism is a system in which all value takes the form of commodities, and thus can be bought and sold in a marketplace.
In another light, it is useful to think of the commodity as the logical opposite of a Maussian gift, and gift systems and the capitalist system as two alternative forms of society. Of course, reality is always more complicated than that, but this is a simplified model meant to clarify different kinds of social and economic practice.
To understand what makes a commodity different than any other object, we have to talk about value. We have to ask what we mean by saying something is worthy, valuable, or good. Many things are useful, but it is hard to compare this value - its use value - to anything else. Shoes are good for keeping feet warm. Mobile phones are good for calling people… umm… Mobile phones are good for texting. How can we express the value of a text in terms of the value of shoes? How shoey is that mobile phone? You see? It's hard to compare different use values because they are qualitatively different things.
And yet people do compare them when they exchange them. Somehow there must be a common basis for comparison. Like Adam Smith, David Ricardo and the other “worldly philosophers“ (classical political economists), Marx argued that what made something valuable was the work that went into making it (Heilbroner 2011 [1953]). Without that energy from work, there would be no phone or shoe, just inert matter. As the rural Chinese woman working in the factory carefully assembled the motherboard of the iPhone, her feet probably had shoes. The shoes made her feet warm and allowed her to work for another hour of her 10-hour day. So you could say that the value of the shoes, not to mention food, clothes, and other necessities, contributed to the value of the phone, because it contributed to the worker’s capacity for labor. Economic activity, for the worldly philosophers, is ultimately a process of channeling and converting the energy of human labor.
Unlike his predecessors, Marx thought that the development of the commodity form was a unique event in history, unlike anything else that had come before. Indeed, gifts are nothing like commodities. When you buy a phone or shoes, you don’t have any obligations to be in a relationship to the person who sells it to you. Buyer and seller are both only looking after their self-interest. What is different about a commodity is that it is an alienable form of value. It is something that can be owned as private property. The worker who created it and gave it life has no claims over it. There is no hau in a commodity.
Put another way, commodities are created under specific kinds of conditions in which a worker sells her labor for a wage. The products of her labor become the property of the owner of the capital—that is, the technology and money—who employs her. This capitalist sells the commodities. He does not have to pay the worker anything more than what other people pay workers; it has nothing to do with the price of the commodity he is selling. The difference between the prevalent wage rate and the price of a commodity is profit for the capitalist. He uses this to buy more capital, to replenish the technological means of production that he owns. So in this sense, the exchange of commodities in a system of capitalism is linked to the system of wage labor, which is for Marx a system in which owners of capital are in a privileged position and can exploit workers.
Thus, capitalism is also a class system. The worker and the owner both participate in the same system and both need each other, but each of them stands in a completely different relationship to the system, and this affects what they see. Owners think of all of this as hunky-dory. They would even say that it is what happens when human beings are allowed to follow their human nature. “Surely, this is the best of all possible worlds!” they say. Owners’ view of the system tend to be dominant. Workers however know a bitter truth about where the value of a commodity comes from.
Heilbroner, Robert L. 2011 [1953]. The Worldly Philosophers: The Lives, Times and Ideas of The Great Economic Thinkers. New York: Simon and Schuster.
Marx, Karl. 1867. “Chapter One: Commodities.” Capital, vol 1. https://www.marxists.org/archive/marx/works/1867-c1/ch01.htm