Not to elaborate too much but think of the relationship with an animal or slave. You don’t pay them a wage, you buy them and their yours and automatically what they do is yours. Their cost is their maintenance like with a machine in its wear and tear.
The same relation doesn’t exist in the working day between a wage worker and capitalist where there is that everyday gap between necessary labor which reproduces the equivalent value for the works reproduction (though Marx analysis doesn’t apply to an individual case as many workers can be empirically below subsistence) and surplus labor which becomes the source of surplus value and underpins profit.
So you seem to get that but onto your part about capitalists trading not at cost value and this seen as them losing out. Marx in some parts assumes the equal exchange of value and treats price and value as the same for the simplicity in some abstractions to illustrate a point.
However, when he does consider an unequal exchange this doesn’t create value it only shifts it. So if when it comes to trade, you might be able to shift greater value around but how does value actually increase?
And while Marx notes exploitation in the difference between wages and labor performed, he doesn’t contradict the idea of equal exchange in his analysis. Its Ricardian socialists who say that workers aren’t getting a fair wage but Marx’s distinction between labor and labour power, as the capitalist can’t own a workers labour unless they owned them as a slave and thus wouldn’t need a wage to entice them to work, is that workers are paid the relative equivalent value of their labour power. Its an illusion to think they were paid for their labor in the first place. Ricardian socialists call for workers to be paid in full, Marx never aims for this. He is more radical in his critique and is about the abolition of capitalism and commodity exchange which underpins the value-form.
And you of course note that many create use-values and that slaves and animals aren’t given money as wages. Where as the worker deprived of a means of subsistence is dependent on commodity exchange. I think its not that they aren’t exploited but not in the way peculiar to capitalist production. Marx’s analysis is i formed by capitalisms distinction to other forms of production but is focused on the particular qualities inherent to capitalism. So commodity exchange pre-exists capitalism but this doesn’t contradict the point that commodity exchsnge is the underpinning factor i. Understanding capitaism and existence of the value-form. Because things are qualitatively changed when the features of commodity exchange become the essential or dominant factor of production. Commodity exchange can be said to be accidental and peripheral to earlier modes of production until it become the essence of modern economies.
However what is fruitful here is slavery in the Americas and particularly the US even as a basis for surplus value.
https://monthlyreview.org/2020/07/01/marx-and-slavery/ In this sense, slavery under capitalism was far more brutal, in Marx’s view, than anything ever seen before in human history. As “the export of cotton became of vital interest to those states [the Southern United States], the over-working of the Negro, and sometimes the consumption of his life in seven years of labour, became a factor in a calculated and calculating system. It was no longer a question of obtaining from him a certain quantity of useful products [as in earlier more patriarchal forms of slavery], but rather of the production of surplus-value itself.”34
...
Key to Marx’s whole understanding of slave-based accumulation in the U.S. South was his notion that, under slaveowner capitalism in its most developed form on “American plantations,” the “entire surplus-value” produced by the slaves “is conceived as profit.… [As] the price that is paid for the slave is no more than the anticipated and capitalized surplus-value or profit that is to be extracted from him” over his working life.53 Unlike the “free wage worker” who has “no value” (as opposed to the value of the worker’s labor power), the “slave…has exchange value, a value” and represents a future stream of value, “a piece of capital.”54 The economics of this meant that the working of slave labor was regulated, as in machinery, in terms of capital consumption, its “wear and tear,” its availability, and the cost of replacements. Nevertheless, the slave, whose initial price was based on a working life of twenty years, was often “overworked,” that is, consumed as a working instrument in seven years, as opposed to twenty, in order to maximize the slave’s production of surplus value in the shortest time. It was also common in this system, Marx emphasized, for slave owners to borrow money upon their slaves as capital assets, hence securities on which to obtain, and to rent them to other capitalists.55 “What Marx…understood,” as Ransom and Sutch pointed out,
was that the slave holding existed to make a profit for the owner. The entire labor product of the slave and family, above whatever provision for food and other necessities the owner cared to make was expropriated. That residual was the owner’s profit and the expectation of a continued flow of such returns made slave property an earning asset. The price paid for a slave reflected the consensus of the buyer and seller concerning the potential value of the continuous stream of profits that could be extracted from the slave and, in the case of a female, from her descendants as well.56
Marx’s analysis thus led him to differ from other political economists and critics of slavery in his time, such as Adam Smith, who argued that slave labor was uneconomic and unable to compete with wage labor.57 In contrast, Marx pointed to the vast surplus labor expropriated from slaves, and the fact that the slaves themselves were a form of capital asset, forming the basis of fictitious or speculative capital.58 Therefore, there seemed to be little doubt, in his estimation, that the plantation economy of the antebellum South was, as far as economic concerns alone were considered, enormously profitable, including the market for the breeding of slaves. As Engels indicated in Anti-Dühring, the reason that only force could remove slavery from the slave breeding and slave consuming states of the South was that production on this basis paid, and thus it would not simply die of its own accord on economic grounds.59
In order to be profitable on a capitalist basis, slave production required a form of production suitable for slave labor.60 Marx explained that the essential element of slave labor was that it was based on force and required continuous external compulsion, requiring the whip of the overseer. Slavery was characterized first of all for Marx by what he called “a relation of domination.” As Patterson has commented in this respect, “Marx not only shows clearly that he understands that slavery, on an institutional level, is first and foremost a ‘relation of domination,’ but identifies the element of direct force which distinguishes it.”61 Because it was directly forced labor, Marx indicated, slaves were engaged in a constant, if not active, resistance. Their laboring conditions lacked their consent; more so under capitalist production where they were forced to work intensively and for inordinate hours, threatening their own corporeal existence. “Forced labour,” Marx wrote, “can never create general industriousness.”62 The resistance of slaves evident in all of their actions, extending at times to slave revolts, and the fear this engendered in their masters, were the primary reasons that it was prohibited to educate slaves, particularly in the South, which meant that they remained almost entirely unskilled labor.
These conditions combined to limit the forms in which slaves could profitably be employed, in comparison to wage labor. Wage labor, Marx argued, was distinguished from slave labor in its flexibility and versatility. Slave labor, in contrast, because continuous force was required, could only be effectively employed in certain forms of production.63 The key limit here, as Marx argued, following Cairnes, had to do with the costs of superintendence. “The greater this [class] opposition” and the greater the degree to which labor has to be forced, Marx wrote, “the greater the role that this work of supervision plays. It reaches its high point in the slave system” under capitalism. Indeed, “the overseer with his whip was necessary to production…on the basis of slavery.”64 Slave labor was uneconomical if dispersed in any way, due to the level of slave resistance, since it would be removed from direct coercion and the whip of the overseer. Nevertheless, slave labor was especially suitable to centralized large-scale production in gangs on monocultural plantations where the costs of the labor of superintendence could be kept down, and where only forced labor could be employed on that scale and with that physical intensity.
Ao it does seem that in colonial slavery rather than ancient because the world market is established they are producing commodities, not things strictly for use value. However their cost is their purchase to own them which is seen in relation to how productive they’ll be over their brief life time of being worked intensely with whip and constant supervision.
So we jave to consider the value of their purchase in relation to what they produce which no doubt is greater than free labor but with limitations also.
BUt then it seems as slaves are treated as objects like machines where there isn't a wage only the initial investment. But against those who deny slaves produces surplus value as in the context of colonialism, it doesn't seem to me that they don't based on quotes from Marx.
https://www.marxists.org/archive/marx/works/1885-c2/ch04.htmWithin its process of circulation, in which industrial capital functions either as money or as commodities, the circuit of industrial capital, whether as money-capital or as commodity-capital, crosses the commodity circulation of the most diverse modes of social production, so far as they produce commodities. No matter whether commodities are the output of production based on slavery, of peasants (Chinese, Indian ryots). of communes (Dutch East Indies), of state enterprise (such as existed in former epochs of Russian history on the basis of serfdom) or of half-savage hunting tribes, etc. — as commodities and money they come face to face with the money and commodities in which the industrial capital presents itself and enter as much into its circuit as into that of the surplus-value borne in the commodity-capital, provided the surplus-value is spent as revenue; hence they enter in both branches of circulation of commodity-capital. The character of the process of production from which they originate is immaterial. They function as commodities in the market, and as commodities they enter into the circuit of industrial capital as well as into the circulation of the surplus-value incorporated in it. It is therefore the universal character of the origin of the commodities, the existence of the market as world-market, which distinguishes the process of circulation of industrial capital.
https://www.marxists.org/history/etol/newspape/internationalist/pamphlets/MARX-on-Slavery-OptV5.pdfIn the second type of colonies-plantations-where commercial speculations figure from the start and production is intended for the world market, the capitalist mode of production exists, although only in a formal sense, since the slavery of Negroes precludes free wage-labour, which is the basis of capitalist production. But the business in which slaves are used is conducted by capitalists. The method of production which they introduce has not arisen out of slavery but is grafted on to it. In this case the same person is capitalist and landowner. And the elemental [profusion] existence of the land confronting capital and labour does not offer any resistance to capital investment, hence none to the competition between capitals. Neither does a class of farmers as distinct from landlords develop here. So long as these conditions endure, nothing will stand in the way of cost-price regulating market-value.
Maybe these sources might point to some possible sources of study.
https://www.ethicalpolitics.org/ablunden/pdfs/For%20Ethical%20Politics.pdf#page90
-For Ethical Politics