As in, was it capitalist or not? I’m happy with a glib and unreflective “it was!” along with “who cares?” This is partly just my gut feeling, and partly from what I read recently about the topic. I hope to have more to say after I read this old stuff published by the Sojourner Truth Organization. (The links under “this” and “stuff” are pieces by Ken Lawrence and Martin Glaberman. Dave Roediger told me and a friend at a conference “Marty always hated it when we or anyone suggested that slavery was capitalist. I think it’s because he didn’t want to conclude that Marx had supported one capitalist country against another.”)

In one of those links, the long quote by Marx about Slavery, is excerpted from a response to Proudhon. The passage is actually ventriloquising what Proudhon isolatees as “the good side” of slavery. Where KM is going with this is a critique of Proudhon’s thought which pits “slavery” and the formal “liberty” of the wage labourer against one another in what Marx was describing as silly bougie hegelianism, one succeeding the other, etc. So the context has more to say on the question you are asking, its form and implications as well as it’s content (is slavery capitalist?).
Here we are, right in Germany! We shall now have to talk metaphysics while talking political economy. And in this again we shall but follow M. Proudhon’s “contradictions”. Just now he forced us to speak English, to become pretty well English ourselves. Now the scene is changing. M. Proudhon is transporting us to our dear fatherland and is forcing us, whether we like it or not, to become German again.
If the Englishman transforms men into hats, the German transforms hats into ideas. The Englishman is Ricardo, rich banker and distinguished economist; the German is Hegel, simple professor at the University of Berlin.
…
Let us see now to what modifications M. Proudhon subjects Hegel’s dialectics when he applies it to political economy.
For him, M. Proudhon, every economic category has two sides — one good, the other bad. He looks upon these categories as the petty bourgeois looks upon the great men of history: Napoleon was a great man; he did a lot of good; he also did a lot of harm.
The good side and the bad side, the advantages and drawbacks, taken together form for M. Proudhon the contradiction in every economic category.
The problem to be solved: to keep the good side, while eliminating the bad.
Slavery is an economic category like any other. Thus it also has its two sides. let us leave alone the bad side and talk about the good side of slavery. Needless to say, we are dealing only with direct slavery, with Negro slavery in Surinam, in Brazil, in the Southern States of North America.
Direct slavery is just as much the pivot of bourgeois industry as machinery, credits, etc. Without slavery you have no cotton; without cotton you have no modern industry. It is slavery that gave the colonies their value; it is the colonies that created world trade, and it is world trade that is the precondition of large-scale industry. Thus slavery is an economic category of the greatest importance.
Without slavery North America, the most progressive of countries, would be transformed into a patriarchal country. Wipe North America off the map of the world, and you will have anarchy — the complete decay of modern commerce and civilization. Cause slavery to disappear and you will have wiped America off the map of nations. [*1]
Thus slavery, because it is an economic category, has always existed among the institutions of the peoples. Modern nations have been able only to disguise slavery in their own countries, but they have imposed it without disguise upon the New World.
What would M. Proudhon do to save slavery? He would formulate the problem thus: preserve the good side of this economic category, eliminate the bad.
Hegel has no problems to formulate. He has only dialectics. M. Proudhon has nothing of Hegel’s dialectics but the language. For him the dialectic movement is the dogmatic distinction between good and bad.
Let us for a moment consider M. Proudhon himself as a category. Let us examine his good and bad side, his advantages and his drawbacks….
just fyi, a good book, despite the title, Mastery, Tyranny, and Desire: Thomas Thistlewood and His Slaves in the Anglo-Jamaican World Trevor Burnard
there’s also in Marx’ letters somewhere a slightly different version of the response to Proudhon:
“Now I will give you an example of Mr Proudhon’ dialectic.
Liberty and slavery constitute and antagonism….”
then the same thing as above, then
“After these reflections on slavery, what will be the good M Proudhon’s attitude? he will search for the synthesis of slavery and liberty, the golden mean, or the equilibrium between slavery and liberty.
“M Proudhon has well understood that men produce cloth, linen silks, and it is a great merit that he has understood so much! What M Proudhon has not understood is that men, according to their faculties, also produce the social relationships in which they produce the cloth and the linen. Still less has he understood that men, who produce social relationships in conformity with their material method of production, also produce ideas and categories, that is to say, the abstract, ideal expressions of these same social relationships.”
to Pavel Vassilyevich Annekov Dec 28, 1846
(i restore the context here for your consideration in light of your other inquiry into the genesis of such features as historicising reflexivity in marx’ method of textual critique. He goes on to emphasise the social conditions and interested tendency of Proudhon’s interpretive practise.)
i don’t know anything about this debate but i thought you might be interested in this article if you haven’t seen it. (i think it is a bit silly, but it is also interesting.)
david graeber, “turning modes of production inside out: or, why capitalism is a transformation of slavery,” critique of anthropology 26(1).
http://coa.sagepub.com/cgi/content/abstract/26/1/61?ck=nck