Isn't this the wrong question, though? Maybe in some hypothetical world of free exchange and no force, there would be no exploitative class - but, getting a system of "free exchange" _in the first place_ requires force. One of the main things that Marx is trying to show in volume one of _Capital_ is that free exchange depends on approaching the world and other people in particular ways (seeing diverse things as being capable of exchange as equivalents), and that approaching the world in this way has certain material presuppositions (especially, making all human labor power equivalent and exchangeable).
According to Marx's analysis, free exchange requires that workers be considered as separable from the means of production (otherwise, you couldn't have formally free labor, which underpins free exchange), and, furthermore, this separation has historically been accomplished by force. Hence free exchange has, as a precondition, the existence of an exploitative class.
"The immediate producer, the labourer, could only dispose of his own person after he had ceased to be attached to the soil and ceased to be the slave, serf, or bondsman of another. To become a free seller of labour-power, who carries his commodity wherever he finds a market, he must further have escaped from the regime of the guilds, their rules for apprentices and journeymen, and the impediments of their labour regulations. Hence, the historical movement which changes the producers into wage-workers, appears, on the one hand, as their emancipation from serfdom and from the fetters of the guilds, and this side alone exists for our bourgeois historians. But, on the other hand, these new freedmen became sellers of themselves only after they had been robbed of all their own means of production, and of all the guarantees of existence afforded by the old feudal arrangements. And the history of this, their expropriation, is written in the annals of mankind in letters of blood and fire." (Marx, _Capital_, ch. 26 http://www.marxists.org/archive/marx/works/1867-c1/ch26.htm )
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"Boredom is the threshold to great deeds."
-- Walter Benjamin