Cross-posted from the World Socialist Party of the United States website
Marx begins his examination of capitalism in Capital with the analysis of the commodity; and he succinctly explains the reason for his starting point in the first paragraph:
“The wealth of societies in which the capitalist mode of production prevails appears as an ‘immense collection of commodities’; the individual commodity appears as its [wealth’s] elementary form. Our investigation therefore begins with the analysis of the commodity.” (Penguin edition; p. 126)
The term “wealth” is used here not to refer to riches in the form of money, but rather to the material wealth essential to any form of society: the products produced and consumed by human beings to sustain and improve their lives.
Material wealth is the basis for the existence of any society, including socialism, but only under capitalism does wealth overwhelmingly take the form of products that are bought and sold on the market; which is to say, only under capitalism does wealth appear as a “vast accumulation” or “vast heap”  of commodities.
Commodities of course existed prior to the emergence of capitalism, as did money, but it is only under the specific mode of production of capitalism that the bulk of material wealth—the vast majority of products of labor—take the commodity form.
It is true that some things produced under capitalism are for the direct consumption of the producer rather than for exchange (like tomatoes grown in a backyard garden), but such products are clearly an exception to the prevailing commodity production. A quick glance down the aisles of Wal-Mart is enough to confirm Marx’s observation that capitalist wealth appears as a vast accumulation of commodities. It was true in 1867 when Capital was published and is even more the case today.
There is nothing arbitrary, then, about Marx’s starting point, as it is based on two undisputable facts: (1) material wealth must be produced in any form of society; (2) material wealth overwhelmingly takes the commodity form in the specific case of capitalism.
Marx is not starting with some abstract concept that only exists in his own mind, but rather with an objective observation concerning capitalist society. (We will see later that nearly every misinterpretation of Marx’s “labor theory of value” is based on an inability to appreciate his starting point in Capital.)
In examining capitalism, Marx naturally concerns himself primarily with understanding the specific functioning of this mode of production, which sets it apart from other forms of society. So he begins with the examination of the commodity as the specific form of products of labor under capitalism or “elementary form” of capitalist wealth.
The defenders of capitalism, in contrast, attempt to present the commodity and other economic forms of capitalism as if they were common to any form of society; which is to say, they try to portray capitalist production as synonymous with social production in general—or argue that it is a reflection of some unchanging human “propensity to truck, barter, and exchange one thing for another” (Adam Smith).
That first paragraph of Capital teaches (or at least reminds) us though that the commoditization of wealth is only a widespread phenomenon “in those societies where the capitalist mode of production prevails.” There is no iron-clad law, in other words, that products of labor need to pass through the market before their useful qualities can be enjoyed.
The “unchanging law” of human society is not commodity exchange, or the need for money to mediate that exchange, but rather the simple fact that material wealth must be produced and consumed in order for a society and the people within it to continue to exist.
There are specific reasons—specific social or class relations—that account for why products of labor take the commodity form under capitalism (as Marx explains later in Chapter one). And once we understand those reasons why commodity production is necessary under capitalist social relations, we can already begin to understand what sort of different social relations will be necessary to move beyond the historical stage of commodity production.
The fact that we are so immersed in capitalism, however, where we have to pay for almost everything we consume, often blinds us to this simple historical fact that human beings under feudalism and other pre-capitalist forms of society managed to produce and distribute (the bulk of) material wealth in a way that did not involve buying and selling.
Marx overcomes this capitalist-induced myopia by treating capitalism as one historical mode of production (among others), which functions in specific ways under its specific social relations. He teaches us to distinguish between what is general to any form of society, and what is specific to capitalism—as in the distinction mentioned earlier between “material wealth” and its “commodity form.”
 This is from the translation of Hans Ehrbar, available on his website (http://www.econ.utah.edu/~ehrbar/akmc.htm). Ehrbar has translated Capital and other works by Marx; and in the annotated version of his translation he guides the reader through the text, while pointing out many of the mistranslations of the Penguin and International Publishers editions of Capital. His website is an essential resource for anyone interested in studying Capital.