First off, let's imagine Marxism as Marx imagined it. Everyone has become the "New Socialist Man", and is no longer interested in their own life, wants, needs, dreams, or aspirations, but cares only about the almighty State and what they can do for their government. There is no money. There is no market. There is just whatever amount of stuff happens to have either been stolen from the bourgeois (capital) and the land itself. The running-dog capitalist exploiters of the noble workers and virtuous peasants and their itching-and-scratching-dog lackeys have been heroically tortured into submission or justly executed along with their families. It is time to celebrate our glorious revolution and to proceed to enjoy our new workers' paradise.
There is a problem, however. We have abolished money! We have abolished the market! We have no way of determining if anything is more or less valuable than anything else! So how do we decide what to produce? The answer is that in the absence of a free market, we have no way of determining whether ten tons of iron ore (about two tons of iron), 200 pounds of rubber and plastic, 50 pounds of glass and the labor, equipment and facilities required to turn the forgoing into an automobile are worth more or less than the automobile we could produce from them. Moreover, we have no way of comparing the value of ten of those potential automobiles to the value of one railroad engine. Or of five tractors. Or a refrigerator (which requires some different materials, and some of the same materials). Or a computer program (which requires almost none of the same materials and a totally different type of labor). We have no idea what we can do to serve our fellow man, which we want oh-so-badly to do, because we have no idea what he values.
So, with heavy hearts, we have to return to the drawing board. But do not lose faith, comrades! Just because Marx's dream of a moneyless and marketless economy does not stand up to even casual scrutiny, that doesn't mean that he was wrong. He just wasn't right. And logic, right and wrong are concepts of the bourgeois mind anyway. Our superior proletarian minds will find a way to overcome these temporary setbacks on our glorious revolutionary road to freedom, democracy, and the mass murders of all those who disagree with us.