I just read the article you linked. It's full of utter nonsense; it is not serious economics. One example: it talks at length about how there are different forms of value, namely individual value (which any normal economist would call production cost), and social value (the actual amount the commodity sells for). It then goes on to say, "there are two basic forces that govern the way individual values become social values" [emphasis mine]: (A) average productivity, and (B) interaction of supply and demand. The first is described mostly correctly. When describing the second, the article says, "We only learn how much labor society has put into widget making when we enter the market and compare the products of our labor with the rest of society." Okay, but that is not the only thing we learn from the market price. We also learn whether the society needs our commodity at all. Since the article talks about "social value" it almost implies that our commodity will always have some value to the society (it's called "social" after all), but that is often not the case at all.

But here is where the article is most clearly wrong:

> But most important to the MudPie theory, demand doesn’t create the social value of a commodity. It only helps determine if labor has been apportioned to the right tasks. Labor is creating the value

First, the article is contradicting itself. It reduces everything to "labor is creating the value" -- if that were the case, why mention "governing" force (B) above at all? Also note, which value are we now talking about? Individual or social? (I assume the latter). Second, it conveniently presents the MudPie argument as "belief that demand is creating value". No, that is not what the MudPie argument says at all. It is the recognition of demand (which is an intellectual process) that is creating value, and that value can be (although not always is) wholly separate from the labor cost (or the "individual value").

Consider the following example. I buy a used or antique piece of furniture from someone who used to own it and who now believes the piece is quite useless since it is old. In addition, the owner does not mind selling me the piece at a low price since he had got out from the 20 years of using it all the "value" he believed he could get out of it. On inspecting the piece, I determine that the piece was made by a known designer. I resell the piece for ten times the amount I paid for it, and for three times its retail purchase price when it was new. Where is the labor here (ignoring transportation cost)? The answer is that the "labor" here is in me recognizing the potential value of the piece either from the appearance or through matching the label through a catalog (about 5 minutes of work). Now exactly the same process will occur when a reseller of a commodity finds a more profitable way to sell it. The reseller is creating value.

TL;DR There is also intellectual labor involved after the item is made but before it is sold, ignored by Marx entirely and purposefully.