It's not a valid argument against the fact that everyone does have their marginal benefit and marginal costs for every good and service that can translate to a binary decision of whether they would purchase a good or service at a given price.

Just because you can't see it doesn't mean it's not there. And I'm not saying it's impossible to find out individuals marginal benefits and costs, because in markets you very well can. Stock traders, when evaluating whether a stock would go up or down, would often buy a bunch of stock, see how the market takes it, then sell a bunch, and by measuring how fast the market eats his orders, he can estimate the aggregate marginal benefit and cost of holding the stock, and then speculate accordingly.

I think this fact is more like an axiom.