If it involves "an oligopoly, a cartel, or a monopoly" then it is not a "free market" according to what they taught me in econ 101, everything convincing that I've heard since, and what Adam Smith explicitly wrote down when he first described the idea. Wikipedia cites Karl Popper in saying that in classical economics a free market is one that's "free from all forms of economic privilege, monopolies and artificial scarcities," and that it's a market in which economic rents are minimised. A monopoly is by definition antithetical to a free market. Any neoliberal suggestions that attacking the whole concept of public regulation of markets will always make them more free are simply lies, and should not be accepted.

That there is at present little or nothing preventing any imperfectly but approximately free markets that might otherwise exist devolving into less free ones dominated by monopolies, cartels, corrupt and captured regulators, out-of-control rent seeking, frauds that rely on information asymmetry, and other such perversions is (obviously, I thought) the reason why I've been consistently saying that "free" markets are not something we see much of in reality. Perhaps that's not exactly congruent with Marxism, but I don't think it's inconsistent with it either.