Marx always begins describing things in Das Kapital simplifying, and then, in later chapters, expanding the model, concluding that in fact it was more complex than was previously presented. The market value of some item would be proportional to the labor only if you consider an isolated industry as an almost closed system, like Marx did in the first book. At the third book, Marx shows that competition and search for profit would produce a force that equalizes the profits for several industries. In the end, prices cannot anymore be explained by the "value" of the product expressed in labor: several goods will be sold above or below their "value". Even if this value is still the origin of the surplus value that creates profit and growth in the capitalist economy.

Marx theory of value is much more elaborated than the one from Smith and Ricardo, and most arguments against it, are in fact arguments against naive and simplified versions of the theory.