[Alan]

Yeah, I thought that article was going to explain marginal utility and how marxism accounts for it but it just attacked marginalist economists without explaining the theory.

Basically, classic liberal economists like Smith and Ricardo, and the younger Marx, believed that the value of a product is directly tied to labor.
Thus if a pound of sugar takes two hours of labor to be processed, a pound of sugar should cost the same as the price of two hours of labor.

But we know this is wrong, and it's best explained by the paradox of water and diamonds. Diamonds are much more expensive than water when water is a lot more essential to us, and if anyone were just lucky to find a mine, he would need very little labor to become immediately rich.

Marginal utility fixed this apparent paradox very simply. All profits are but the value of marginal utility, meaning the difference between the purchase and sale of a product b playing with supply and demand. A product has no value in itself but has only relative value depending on how much the product is desired and how scarce or abundant it is. Same for services.
This also goes against the early Marxian concept of profit as surplus labor and the Lasallean Iron Law of Wages.
The iron law of wages is wrong, capitalists don't need to pay the bare minimum to workers to make a profit, they just need to manage the market to make a good marginal profit.
Equally, the profit of the capitalist is not surplus labor, meaning the part of the worker's labor that the capitalist refuses to pay to the worker (e.g. imagine a worker is paid 50 bucks a day, but in that day he makes 100 bucks worth of, let's say, microchips; the idea of surplus value is that the worker effectively gave 50 bucks of his labor to the capitalist for free) but instead it's merely the purchase of a workers' labor power at a negotiated price and the acumen to sell the product of this labor power at a higher price than the price of the labor power.
Thus, a capitalist is not like a slaveowner paying workers for certain hours of work but demanding that they also work extra hours which the capitalist pockets, but rather a very shrewd merchant who buys a product or a service at some price and finds a market in which he can sell it at a profit.