Commodity exchanged for money or C-M is selling. Money exchanged for Commodity is the act of buying. Remove money from the picture is it’s basically exchanging one Commodity for the other, or C-C. The circuit ends once the seller becomes a buyer of a commodity.

Money exchanged for a commodity to sell that commodity to advance your money is M-C-M. So technically it would be better represented by M-C-M’. Your money is used as the thing used to accumulate commodities, you then sell those commodities to receive back money advanced or more money.

The first circuit is you sell to buy, the second circuit is you buy to sell. M-C-M is what makes the capitalist, C-M-C is what makes the consumer. The first sells what he produces to consume other commodities, the second buys commodities to sell them and accumulate more commodities to sell them again aka accumulation of capital.

I simplified it but that’s the gist of it.