"Contradiction in the capitalist mode of production: the labourers as buyers of commodities are important for the market. But as sellers of their own commodity--labour-power--capitalist society tends to keep them down to the minimum price."
An unusual recognition by Marx of the importance of the working class as a source of consumer demand, giving it quite a 'Keynesian' flavor along the lines of the paradox of thrift or other paradoxes of macroeconomics.
It reminds me of a clever editorial cartoon during the 1990 recession: the picture shows a number of factories, all connected to one another by roads. Each factory is saying: "We don't want to hire people, we just want to sell them stuff!"