Part Four — The Transformation of Commodity Capital and Money Capital into Commercial Capital and Money-Dealing Capital (Merchant's Capital) Chapters 16–20. Commercial capital, commercial profit, its turnover, money-dealing capital, and the history of merchant's capital.

Part Four analyses the first of the great functional differentiations of the total capital: the splitting-off of commercial (merchant's) capital, which takes over the buying and selling that the industrial capitalist would otherwise do himself. Two forms are distinguished — commercial capital proper, which deals in commodities (the commodity-capital phase C′–M′ made independent), and money-dealing capital, which handles the technical operations of money (payments, receipts, the management of the reserve). The central problem is a value-theoretic one, and it is the same problem the whole volume keeps posing in new forms: the merchant plainly makes a profit, yet buying and selling create no value. How, then, is commercial profit possible without breaking the law of value? Marx's answer is that commercial capital produces neither value nor surplus-value but shares in the surplus-value produced by industrial capital, drawing the average rate of profit on its own advance as a member of the total capital. Commercial profit is therefore a deduction from industrial surplus-value, a redistribution, not a new source. Chapters 16 and 17 establish this; Chapter 18 analyses the turnover of commercial capital and its effect on the merchant's mark-up; Chapter 19 treats money-dealing capital; Chapter 20 gives the long historical excursus on merchant's capital, which existed for millennia before the capitalist mode of production and whose autonomous power, Marx argues, stands in inverse proportion to the development of that mode.

Commercial capital is the commodity-capital phase of industrial capital given independent existence in a separate set of hands. The merchant advances money to buy commodities from the producer and sell them to the final buyer, performing the metamorphosis C′–M′ on the producer's behalf. Marx insists on the value-theoretic consequence: this activity, being pure circulation, adds nothing.

The definition is given at the outset: commercial capital is a portion of the total social capital, permanently resident in the sphere of circulation.

Commercial capital, then, is nothing but the transformed form of a portion of this circulation capital which is always to be found on the market, in the course of its metamorphosis, and perpetually confined to the circulation sphere. We refer here to a portion only, because another part of the buying and selling of commodities always takes place directly between the industrial capitalists themselves.

Marx, Capital III, page 380.

Commercial capital thus creates neither value nor surplus-value, at least not directly. In so far as it contributes towards shortening the circulation time, it can indirectly help the industrial capitalist to increase the surplus-value he produces.

Marx, Capital III, page 392.

By specializing circulation in one place, commercial capital reduces the total circulation costs and circulation time of the whole system, freeing more of industrial capital for production. It thus indirectly raises the mass of surplus-value produced, and it economizes the money and labour tied up in buying and selling. But none of this makes the merchant a producer of value; it makes him an agent who allows more value to be produced elsewhere, in exchange for a share of it.