At the very least, though, the concept of a “market” should require the voluntary exchange of commodities, shouldn’t it?
I don’t think so. All monetary exchange involves aspects of power, so in that sense, it is never entirely or even mostly voluntary. But it is never only about power either. In a company town, the power of the company is significant, but not absolute. If it were absolute — like in a slave plantation or a concentration camp — there would be no need for monetary exchange.
The capitalist mode of power is mediated through monetary exchange. And if exchange is a vehicle of power, you cannot assume it away. In this sense, capitalism sans markets is an oxymoron.
But I could be wrong.