The analogy had nothing to do with any legal implications. It is a simple matter of non-adventuring persons in a fantasy world having better things to do with their money than buy gear that they won't be using. This leaves very few people (adventurers) in the population to support a magic item economy.
The analogy has everything to do with any legal implications.
In the real world, the consequences for walking around with heavy armor, heavy weapons, and breaking and entering, murdering, and looting are very harsh. There are virtually no places on the planet where this is acceptable behavior. Hence, there is a very limited and illegal market for most of the more powerful weapons/armor in the real world. And guess what? The market for arms STILL exists in the real world, it doesn't matter if they cost millions of dollars.
In most fantasy worlds, the consequences only happen if the PCs do it to NPCs that are in a village, town, or city. Once outside of civilization, it's all fair game (and even sometimes inside of civilization with certain monster and undead). Hence, there very well could be a legal market for many of the non-artifact (or maybe non-legendary) weapons/armor. In addition, wealthy NPCs would certainly buy some magical weapons and armor for their guards. In a world of adventurers with magic spells and items, it only makes sense that wealthy NPCs would protect themselves. That's a legal market that would exist. Wealthy NPCs would also want PCs to go out and kill monsters and other threats to society. Adventurers do that stuff in a lot of cases for free. So, wealthy NPCs wouldn't be opposed to adventurers in a fantasy world buying, selling, and owning magic items, as long as the adventurers are out killing monsters and not attacking the wealthy NPCs. Once adventurers start doing that, the laws would quickly be changed.
Hence, it's a very poor apples and oranges analogy. The legal implications make these two settings totally different.
Perhaps if there were numerous items of magic that would help normal folk such as brooms of sweeping, hammers of forging, towels of cleaning, and so forth and these items were inexpensive enough for a good potion of the populace to afford them then a magic item economy would be more likely, with some shops having items of interest to adventurers in addition to the bread and butter items.
Like Eberron?
The idea of magic shops all over the place carrying only adventuring gear for the few adventurers in the population isn't a sustainable model for the merchants.
Maybe not magic shops that just cater to adventurers, but having magic items as one of dozens or hundreds of other items being sold by merchants in a world where PCs and NPCs can craft these valuable items is a sustainable model. Like in the real world, valuable items are protected (possibly in vaults) and arrangements are made for the sale of said items.
Which merchant is more wealthy? The one who sells one 1000 GP magic item a year, or the one who sells a thousand 1 SP items a year? Value equates to greed and commerce. If there is a market, somebody's going to try to make a buck off it. It's how economics work.
Saying that NPCs wouldn't buy and sell magic items in a fantasy world is implausible. It's totally against human (and presumably humanoid) nature. It's called greed and it should exist in a fantasy world.
Now, I can understand this being a black market in a given DM's world, but not existing at all? That doesn't make sense at all. Even in a world where magic items are extremely rare, there would be wealthy NPCs with nothing to do with their extra money, trying to buy them.
