There are many things in this world that are exceedingly rare or unique, whose existence at one time or another was either thought legend or lie, and in times far preceding the existence of the information gathering tools that exist now.
None of which is a counterpart to a magic item. Being exceedingly rare or unique is a common trait, and indeed things with only those traits would obey normal supply and demand.
For all of human history, in any culture you can examine, trade in things humans THOUGHT were magic or otherwise supernatural- potions, talismans, artifacts of the divine, holy relics, etc.- has occurred. That magic may not actually exist is immaterial-
Entirely wrong. The actual non-existence of magic is the most salient fact in the real world beliefs about magic. The actual non-existence of magic shaped everything that people believed about magic and how they behaved toward it, regardless of whether they actually believed in it. The real world being exactly like it was, but also containing magic, is a fantasy that can only occur in stories regardless of whether or not you believe in those stories. Just as the existence of Superheroes would profoundly alter society so that the comic books world of being almost exactly like the real world but with Superheroes can't exist outside of a comic, so the sort of magic people believed in could only exists in the stories people told themselves. The people in the real world believed that they lived in a world of magic, but since magic wasn't exactly an everyday experience, believed that magic was rare, that it required special people to perform it, or that it was almost non-existent here but that they'd heard reliable reports that it was prevalent elsewhere. Dragons always lived in the unknown spaces of the map.
One of the most important aspects of a running fantasy RPG is dealing with trying to create verisimilitude to stories about magic created in a world were magic didn't exist (or expectations of same) within a world were it is actually available (or is expected to be). The same could be said of a Supers game. You have to be careful to construct the setting in such a way that you don't break the box even though what is in the box isn't what created the narrative it tries to create.
the purchasers (and sometimes the sellers) entered into transactions with the belief in the reality of magic or the supernatural, and that shaped their decisions as actors within RW markets. People made decisions to spend their money on a sacred ritual instead of fertilizer, on love potions instead of hygiene products, on sacred talismans instead of body armor.
Absolutely. But for example you mention the relic trade, where you could produce and trade fakes as commodities - enough splinters of the true cross to shingle a barn, for example. But supply isn't the overwhelming problem here. Real relics wouldn't have been commodities for reasons that don't just depend on their supply. The structure of the trade is entirely based on the fact that the fakes were commodities. Real relics wouldn't have obeyed normal rules of supply and demand. The fakes could because they could be manufactured, priced, and parted with. There might be no economic difference between buying a magic sword and buying a sword that you only think is magical, but there are real differences when one is actually magical.
The reason why "Its magic" is sufficient to explain why a cloak can make one fly like a bat is that the maker made it so, using the supernatural physics of the world. But what maker decides to create items that do not follow the rules of economics?
For example, a maker who decides to create an item which can never be used against him, and so makes it so that the item works only for himself (and perhaps curses anyone else). Ironically, in many magical creation systems, such a restriction reduces the cost of creating the item (which creates the problem, why don't the vast majority of items have this restriction?). In a sense, the example item does obey the rules of economics, in that the demand for this item is now effectively zero to anyone but its supplier. But in the sense you mean it, that is that magic items are commodities and so they must behave like normal goods, then no, it doesn't obey the rules of economics.
Despite the fact that they weren't magic items, the relic trade in some senses actually proves my point, because the more people actually took the relics seriously the less like commodities that they actually behaved. The relic trade was dominated by sellers producing in the middle east that did not believe the relics were real or even if real were in any way magical, and buyers from Europe unable to evaluate the sacredness of what they bought (and in some cases not caring). But once the relics made their way back to Europe, normal economics is largely suspended. On what price can you place the hand of the divine? Once you had it, was it irreligious to actually part with it? The relic trade was dominated in Europe by the lack of trade. Once in Europe they were no longer for sale. They were priceless artifacts capable of irreplaceable good. Once acquired from the heathens, it was considered disrespectful to purchase one or sell one. They could only be preserved and venerated. The more seriously the relic was taken, the less likely a price was assigned to it. In fact, any attempt to actually sell a relic by a town or church was met with complete disbelief to the point of disinterest. If a town attempted to sell a relic, the presumption of all buyers was that either the town had discovered the relic was false or else that something had happened to it to defile it and remove its powers. No one believed anyone would willingly part with something actually sacred and puissant since if it remained sacred and puissant surely it could supply abundantly whatever need it was that you wanted to fill by selling it. Once recognized as a relic in Europe, the item was removed from normal economics because its price was effectively infinite (and thus its demand effectively zero). The closest they came to being normal goods was as positional goods - if the price was less than infinite, then it was presumed the actual value was zero. If it was for sale, it wasn't a relic. Only heathens would actually sale an item.
Ironically, this meant that what comparatively little post-purchase relic trade existed in Europe was based on theft.
Of course, this itself was based on the fact that the magic wasn't real, since if it was real, then presumably no one would dare attempt to steal one either for fear of divine retribution. The fact that theft became popular was also itself based on the fact that the theurgy didn't really work regardless of whether the thieves believed it was. Only the combination of belief and lack of reality explains the economics.
If the magic had actually been real and effective and demonstrably so, these factors wouldn't merely informed trade in the items, they would have absolutely governed it. There would have been only one foreskin of Christ, or jar of Mary's Milk, (or pick your favorite superstition), and it wouldn't have had a buyer, because though everyone would have wanted it, no one would have dared transfer it for cash, and it wouldn't have had a seller because the item would have been priceless, irreplaceable, and imposed an obligation on its owner.
And that is to not get into the fact that to a certain extent the belief in the thing can be a commodity is a prerequisite for something behaving like a commodity.