Karl Marx. Capital Vol One Ch5

Contradictions in the general formula for capital

Making money from money presents itself some problems. If you sell something at equal value than what you paid for it, there is no room to profit. Even if you buy a raw materials and sell the product for more than the materials, you are adding the cost of labor.

A profit does not seem possible from the circulation of commodities. You will just the equivalent back. This is good if you want to exchange a commodity but troublesome if you are expecting to make money into more money.

Marx supposes that a profit must be made within the circulation of commodities and out-side of the circulation circuitry.