It is the merchant who has definitively sold the commodity. The merchant becomes part of the circuit, one of the steps completing the process. The commercial capitalist is an independent extension of the producer, a role that could theoretically be met by an agent of the producer, but only inefficiently for the real system as a whole. This money exists and functions as commodity capital. Note again that Marx is talking about the reproduction of the whole social capital.
Commercial capital is nothing more than capital functioning with the circulation sphere.
The circulation process is one phase in the reproduction process as a whole. You can already see, Rebecca, how you have conceptually and erroneously lifted commercial capital out of its relations. The same value simply undergoes changes of form. This point is crucial. Surplus-value, according to Marx's theory, can only be generated in production, through the exploitation of some labor input whose productions are destined for capitalist markets.