: Doesn't the characterisation of capital as M-C-M' force us to term as capital certain production relations that have no relation to the capitalist mode of production (e.g. merchants' capital)?
Got to pay attention to the C. That's commodity as in the production of it.
... [M]erchants' profits sprang from circulation (i.e. without production of surplus value).
Surplus value is not a capitalist invention. It existed then.
Now as then, merchants receive a part of the surplus of a commodity for providing the producer of the commodity with its final sale. What's different is who is the producer! Now it is---more and more---the production capitalist who controls the labor-power of others. Before, the simple producer had to surrender to the merchant some of his or her surplus in order to find a market.
The value is always in the production. It cannot be in the circulation. See Marx's refutation of 'Say's Law.'