I find this a curious statement. What is inappropriate about assuming magic items follow the rules of econimics, namely, that people will try to buy and sell them?
I don't know that it is 'inappropriate', as you could certainly create a setting where it would be appropriate.
But there is nothing that forces magic items to follow the same economic rules that govern onions or bricks.
a) Magic in most settings isn't technology. This most easily scene by trying to create a wizard analogue in a science fiction setting without resorting to magic in the form of psychics, psionic, and so forth. Anyone with small training can use a gun or guided missile. It's less obvious that wands, spellbooks, crystal balls, and so forth are transferable and in some settings they explicitly are not.
b) At the least, magic requires highly specialized knowledge to even appraise. You could look at most pieces of technology and have some idea what it was supposed to do, presuming it wasn't several tech levels beyond you. But magic items generally can do anything and have very few clues inherent to them as to what that anything is. Magic makes the art of the con so easy, and many stories about magic are premised on that, that I imagine most people would be afraid to buy anything magical unless they were highly skilled in the magical art themselves.
c) In general, magic is not amendable to mass production techniques. In general, magic is scarce compared to say televisions or sneakers (and if it isn't, then it's likely magic as technology). In practice, for most people magic is irreplaceable. If they have magic at all, it's as a heirloom which is not only useful beyond anything else they might own, but which has significance of heritage and purpose above and beyond its monetary worth. Aragorn doesn't sell the shards of Narsil, not just because they will be useful to him, but because they are in some sense who defines him. Just as the PC's will likely sacrifice everything material they have but last off all their irreplaceable magic items, I imagine most NPCs - who have been attached to their items for far longer - will give up everything before selling their items. The items are not merely useful now, but as the foundation for future wealth and restoration of position, and as a reminder of where they are. To give them up is to admit you are a nobody.
d) In general, magic is weaponized. Weapons aren't generally subject to the same economic considerations as other commodities. You might, in some nations, be able to purchase a simple weapon for self-defense. But in pretty much all nations above the level of failed states, some weapons are not on the open market. There are no stores you can walk in to buy Hellfires, Claymores, and Abrams tanks. Most nations probably would object to plains to purchase and import 40 T-80 main battle tanks for private use. It's strange enough that D&D citizens regardless of rank or station seem to be perfectly free to purchase plate, warhorses, swords and lances. That they being less than nation states could purchase arbitrary magic items outside of lawless lands stretches my credulity.
I tend to do a hybrid based on demographic assumptions. Assuming that NPCs almost never reach about 5th level at the pinnacle of their careers, but that levels between 1st and 5th are fairly common, there is in the campaign a background level of magic as technology in magic that is accessible to low level characters. And to a large extent, society is built around that assumption that magic is in some sense normal. But there is a sharp divide between say a potion, which ordinary craftsman can create, and the sorts of magic items that can only be crafted by rare and semi-legendary figures. These are seen as belonging to greater mysteries and greater arts. You might easily go into a market and buy or sell a 'healing elixir to knit the flesh', but there exists no ordinary market for 'Excalibur'.
