As for the actual question: Marx's law of value pertained to commodities, freely reproducible objects (and so they are not original artworks, unless you have devised a scheme by which "original art" is itself a commodified service to be assigned a price tag, which in a sense is something we have done… and as a result, commissioned artwork has a fairly low going price, dependent on the availability of starving artists who will create furry porn, and ignoring for a moment that furry porn is produced by a cartel of sorts that fixes prices for their own benefit and recognize that they shouldn't undercut their fellow furry porn creators).
Nearly everything about "luxury" goods is explained by the prevalence of cartels and price fixing, which would be the thing Adam Smith calls "a conspiracy against the public, or some contrivance to raise prices". People often try to forget what Adam Smith was really describing, and sometimes Marx himself is doing this or willfully ignoring what political economy entailed.
It should also be remembered that Marx describes the law of value to explain how political economy was nonsensical on its own terms; that if you actually did this, you are missing a lot of very relevant details about what actually happens in capitalist society. Nothing in the free trade theory mentions primitive accumulation, which is why Marx spends chapters describing this process.
Generally though, the reason free trade is allowed, the reason why this system can work, is because price fixing is mitigated and the producers are competing to provide goods for the lowest price, without concern for any external want. Obviously if you can produce goods for cheaper than your competitor, you have an advantage against them. You can sell your goods at the same price as your competitor, who can't do shit against you except try to emulate your ability to produce the same good for cheaper. So, if some starving artist says "I will create furry porn for basically free, as long as I receive a diet of Hot Pockets and am free to make you more furry porn", he's going to have an advantage over a competitor whose needs are greater, say if he has a family to feed. You can see where this heads, once society has degraded enough that the family and even the most basic expectations of human existence can be cannibalized. You're never going to compete with people who live on practically nothing, are used to living on nothing, and have no expectation
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