Duncan, replying to Gil.
I accept your correction that Marx used the phrase "capitalist
exploitation" to refer to the extraction of surplus value in the form of
capital (for example, usury) but on the basis of social relations other
than the wage-labor form.
I still think it is useful analytically to separate out the form of
surplus value (rent, profit, interest) from the social relations through
which the surplus value is exploited.
Still, it seems to me that Marx's pedagogical aim in the first part of
Volume I of Capital is to address someone familiar with Ricardian
economics, and to explain his view of exploitation in the language of
Ricardian value theory. Where he's heading is the distinction between
labor and labor-power and the idea that labor-power becomes a commodity.
He's at pains expositionally to distinguish his view from the commonplace
idea that capitalists get rich by buying cheap and selling dear, and he
uses the assumption that commodities exchange at their value to underline
this point.