I think Marx is consistent on his views in this passage and his method of presentation in Volume 1. In the passage, he seems to argue that one has to inquire in concrete level in great detail how the material exists, develops in historical forms, and how its dialectical inner contradiction manifests itself. I think this is the reason why Marx’s analysis is always so rich in history, institutions, real life examples, etc. Then, he says in the passage, only after one inquires at that level, then one can begin presenting, i.e. exposing, the material to the reader. I think this is the reason why he wrote drafts of volume 2 and 3 earlier than volume 1 (correct me if I am wrong).
Why did he begin with his conception of commodity? Well, after reading Aristotle, Aquinas, Petty, Smith, Ricardo, Malthus, Mill, and so on in the past three weeks, and seeing the development of labor theory of value in the minds of these great thinkers, it seems only logical to me to start with a discussion of what commodity is, how the contradiction between use-value and exchange-value exists, and how this contradiction reaches its highest level under capitalist mode of production.
Aristotle clearly recognized this contradiction, but did not ground it in labor theory of value. Smith achieved this, but his conception was dual: labor commanded vs. labor embodied. It was Ricardo who elaborated on the latter conception, which was then taken by Marx to show how surplus-value was created in the “hidden abodes of production” (which was not there in Ricardo’s labor theory of value). I think by beginning his presentation with the concept of commodity, Marx achieves (at least) two objectives: (1) Distinguish between simple commodity production (C-M-C’) and capitalist production (M-C-M’) since the latter requires the product to be a commodity and therefore express itself as money, and through this expression must go through the process of metamorphosis. (2) Labor-power as a commodity: The consumption of labor power creates more value than its purchase resulting in surplus value.
These two ideas seem simple to the regular Marx reader, but very hard to conceive by those who have very little exposition to Marx. And I have experienced that it is indeed a challenge to try to explain these ideas to people who are used to seeing things in a C-M-C world. When the central function of money is seen as enabling the exchange of one commodity for another and not as a means for exploitation of labor power, “there alone rule Freedom, Equality, Property and Bentham” as Marx famously wrote in Volume 1.