It appears at first sight that the merchant buys commodities from a productive-capitalist at their value, and sells them after adding their profit. As a capitalist, the merchant will want to obtain the same average profit on their capital as any other capitalist. They will want a share in the total surplus value produced in the economy, proportional to the size of their capital, and as was seen with all capitals in the formation of an average rate of profit, discussed in Chapter 9.
Yet, in reality, all the merchant is doing here is carrying out the same function, as a separate capital, that a portion of the manufacturer's capital previously performed, i.e. of selling what had been produced. But, as was seen previously, this function added no value to the commodity, and produced no surplus value.
The advantage, for the productive-capitalist, however, can already be seen. Previously, in order for their production to be continuous, they had to advance sufficient productive-capital, not only to cover the working period, but also to cover the circulation period, as was shown in Capital II, Chapters 15 and 16. But, if they can immediately sell their output to a merchant capitalist, at the end of the working period, they no longer have to advance a productive-capital of their own, to cover this part of the turnover period.
The manufacturer produces a certain quantity of linen and sells it to the merchant who uses their own capital to cover the circulation period, whilst it is being sold in the market. In the meantime, the manufacturer has already been paid, their capital has been reproduced, and they can use the proceeds to buy the required productive-capital to continue production.
“But while the sale of the linen, its metamorphosis into money, has taken place for him, as producer, it has not yet taken place for the linen itself. It is still on the market as commodity-capital awaiting to undergo its first metamorphosis — to be sold. Nothing has happened to this linen besides a change in the person of its owner. As concerns its purpose, as concerns its place in the process, it is still commodity-capital, a saleable commodity, with the only difference that it is now in the merchant's hands instead of the manufacturer's. The function of selling it, of effecting the first phase of its metamorphosis, has passed from the manufacturer to the merchant, has become the special business of the merchant, whereas previously it was a function which the producer had to perform himself after having completed the function of its production.” (p 269-70)
The fact that the task of selling the commodities has been taken over by a dedicated capital does not change anything. The producer may have sold their output to the merchant, but, if the merchant cannot sell the commodities, to some final consumer, the merchant will not reproduce their own capital, and so will not be able to buy further commodities from the producer. This is no different than had the producer had to maintain a portion of their total capital as commodity-capital. Until such time as that commodity-capital has been metamorphosed into money-capital, by the sale of the commodities, the capital value it represents could not be reproduced.
Looked at from the perspective of the total social capital, the merchant's capital is nothing more than this commodity-capital separated off as a special function, to facilitate the process of circulation.
“It is, indeed, easily seen here that the merchant's operations are really nothing but operations that must be performed at all events to convert the producer's commodity-capital into money. They are operations which effect the functions of commodity-capital in the circulation and reproduction processes. If it devolved upon the producer's clerk to attend exclusively to the sale, and also the purchase, instead of an independent merchant, this connection would not be obscured for a single moment.” (p 270)