The Profit Puzzle — Das Kapital

Marx now confronts the apparent contradiction head on. If equivalents exchange for equivalents, where does the surplus in M-C-M' come from? He first dismisses the fantasy that capitalists enrich themselves simply by buying below value or selling above it.

In any such cheat, one pocket gains exactly what another loses. The same logic applies to the class as a whole. Capitalists cannot systematically profit by overcharging one another, because they are also one another's customers and suppliers.

Turn and twist as we may, no amount of shuffling inside equal exchange creates new value for everyone at once. Something else must enter the circuit. The chapter ends by clearing the board. Surplus-value cannot arise in circulation while equivalents are traded fairly. Marx has ruled out the obvious escapes. That narrowing forces the next question: which commodity can be bought at value and still produce more value when consumed?