Marginalism just means that the specific, concrete good or thing as the object of choice is the object of people's action and therefore valuation. Marginalism does not require having a market price, but quantitative calculations of accounting magnitudes, like profits, losses, cost, revenue, do require market prices.

I don't know if I totally understand these theories but I think modern market anarchists would say that a "freed" (anti-capitalist) market does use prices, and further strips rent/interest/privilege to make those prices accurately reflect subjective cost of labor as well. A freed market would still need commons rules, democratic decisions, norms, hard constraints, which isn't a contradiction in the system. I've seen Carson synthesize marginalism and LTV, that marginal utility decides price in more short term and competition drives cost to labor over the long term, as under the distorting effects of capitalism, these two are further apart than they need be.