How we cite our quotes: (Chapter.Section.Paragraph) or (Chapter.Paragraph)
Quote #7
In order to extract value out of the consumption of a commodity, our friend the money-owner must be lucky enough to find within the sphere of circulation, on the market, a commodity whose use-value possesses the peculiar property of being a source of value, whose actual consumption is therefore an objectification of labour, hence a creation of value. The possessor of money does find such a special commodity on the market: the capacity for labour, in other words labour-power. (6.1)
How can capitalists make more money when they sell in order to buy if all commodities are exchanged at their value? Marx's project in Das Kapital is to describe an ideal capitalism where commodities are traded for what they're worth. So where does the surplus-value enter the system since all the traders know the scores? Well, there is one special commodity, Marx says, which can create value. That is labor-power.
Quote #8
Given the existence of the individual, the production of labour-power consists in his production of himself or his maintenance. (6.11)
The capacity to labor (labor-power) is primarily used in keeping the individual worker who possesses it alive. This is the necessary labor that must be done in all societies, capitalist or not.
Quote #9
Our capitalist stares in astonishment. The value of the product is equal to the value of the capital advanced. The value advanced has not been valorized, no surplus-value has been created, and consequently money has not been transformed into capital. (7.2.19)
Marx has just finished describing how no surplus-value is created by laborers simply making products during the portion of their working day that constitutes their value—the part that takes care of the cost to keep them alive. Picture a simple tribe gathering berries and eating them. There's no profit, even though they might have instruments of labor (tools) to assist them. So where does the capitalists' surplus-value come from? See the next quotation.