"Nothing could be more foolish than the dogma that because every sale is a purchase, and every purchase a sale, the circulation of commodities necessarily implies an equilibrium between sales and purchases. If this means the number of sales is equal to the number of purchases, it is flat tautology. But its real intention is to show that every seller brings his own buyer to market with him. Sale and purchase are one identical act, considered as the alternating relation between two persons who are in polar opposition to each other, the commodity owner and the money owner. They constitute two acts, of polar and opposite character, considered as the transactions of one and the same person. Hence the identity of sale and purchase implies that the commodity is useless if, when it is thrown into the alchemist's retort of circulation, it does not come out again as money; if, in other words, it cannot be sold by its owner, and therefore bought by the owner of money.This identity further implies that the process, if it reaches fruition, constitutes a point of rest, an interval, long or short, in the life of a commodity. Since the first metamorphosis of a commodity is at once a sale and a purchase, this partial process is at the same time an independent process in itself. The buyer has the commodity, the seller has the money, i.e. a commodity which remains in a form capable of circulating, whether it appear on the market at an earlier or later date. No one can sell unless someone else purchases. But no one directly needs to purchase because he has just sold. Circulation bursts through all the temporal, spatial and personal barriers imposed by the direct exchange of products, and it does this by splitting up the direct identity present in this case between the exchange of one's product and the acquisition of someone else's into the two antithetical segments of sale and purchase. To say that these mutually independent and antithetical processes form an internal unity is to say also that their internal unity moves forward through external antithesis. These two processes lack internal independence because they compliment each other. Hence if the assertion of their external independence proceeds to a certain critical point , their unity violently makes itself felt by producing - a crisis."
Capital I 208-209