I have a friend who is essentially communist, we debate all sorts of things and he generally agrees we need to get rid of capitalism. However he is assured that there is no way to determine the value of commodities as effectively as the market can through supply and demand (aside from his argument that soon an AI super-intelligence can save us without the market incentives). I try to point out that the market as it is isn't efficient and creates massive waste, pointing to various ways that our system generates waste due to capitalism, and he agrees it is extremely inefficient but still the best we can do right now.
I have read a lot more theory about resistance to capitalism, insurgency, organization and the like but not as much about the specific economics of a communist society once we move past capitalism. Where would be a good place to start? Should I just quote Das Kapital to him?
that said, it's probably most fruitful to focus on the "supply" side of things. how are goods and services supplied? does the supply of goods and services ultimately depend on anything else that perhaps provides the true source of value? some kind of socially useful labor time, perhaps? get your friend to the labor theory of value on his own intuition and he'll drop this silly supply and demand liberal economics nonsense.