Marx clarifies the conditions necessary for commodity exchange to occur, chiefly that private property must be a respected right in some way. Independent individuals, independent meaning that their every possession is alienable to the market, must exist. With feudalism we had share-cropping style property, with capitalism we have private property. An important theme throughout Das Kapital is that people are representatives of commodities, that is, they are the personifications of economic relationships between commodities. Here we also see another piece about commodities, that they are not use-values to their owner except as exchange-values. Marx goes on to claim that money is a natural consequence of commodity exchange, it solves the problem inherent in simple bartering, of temporal and spatial issues. Money is an external expression of the differences between exchange-value and use-value, that money can be completely symbolised and not a physical thing at all shows that exchange-value is NOTHING physical, and constitutes a specific social relationship.
Marx takes note of how when exchange first started occuring, it’s pretty clear the differences between exchange-value and use-value. Communities would produce things specifically for trading with other communities and produce other things specifically to be used by their producers.
Marx talks about some necessary properties of the physical money-commodity. It must be divisible and uniform specifically. It goes on to mention that money is merely a manifestation of value with two use-values, the normal physical use-value, and the use as the universal equivalent for all other commodities. This makes it clear that money is a commodity, with a specific value-form. The money commodity goes on to represent ALL human labour.