Most discussions of exploitation in economic and philosophical circles seem to treat it as a species of theft, or, even more diffusely, some sort of unfair distribution of the products of labor. I think this is--as an interpretation of Marx--simply wrong.
Labor is exploited when it is used for an end which is not its own, just as "natural resources" are exploited when they are turned from their own ends to serve human ends. In the case of labor, its own end is to produce some useful good or service. It is exploited when it is subordinated to capital's end--the production of surplus-value. Producing surplus-value presupposes the production of useful things, but entails more than this. Capital exploits labor not by stealing from it, or by keeping more than its fair share of the product, but by subordinating labor's end to its own.
Thus, I think most discussions of "market socialism"--including this one--ignore the specific role of the value form in Marx's critique of political economy. This is the same thing Marx castigated Proudhon for, and so it's no surprise that Proudhon pops up in this discussion. Without addressing the "mongrel" dualisms of use-value/value and concrete labor/abstract labor, any discussion of cooperative production remains on the terrain of capitalism, where it just looks like an inefficient way to accumulate investment capital.