When you're saying "same economic factors" or "economically indistinguishable" what is it that you mean?
I mean that FRPG magic shops follow the same rules of supply and demand as a RW seller of "magic" and potions.
Because their goods are rare, believed to be effective, and so forth, the RW shop price structure and niche in the economy will be functionally indistinguishable from a FRPG's magic store.
Given analogous societies across the RW/FRPG barrier, the demand for magic is the same. Similarly, the supply of such items will be restricted, especially since- real or fantasy- the knowledge and materials involved in such commerce are not (usually) common.
in addition, the assumption that every DM who permits the sale of magic items in ther campaigns does so based on the modern strip mall/department store model is incorrect. That economic model presumes that magic items are relatively common, so not everybody allows that.
Instead, based on prices alone, most major magic items should be
at least as rare as a pretty decent suit of armor- chainmail or better- and craftsmen of that skill are not common.
There are 2 magic item sales models I personally use*:
1) regular merchant who has something magical to sell. He will not have a broad or deep stock of arcane items, and what he has will usually be a magical version of something he would normally sell- so he has some idea of its worth- but he may or may not be aware of its magical nature. Thus, a bargain may be had, but you may also get ripped off...knowingly or unknowingly.
Rarity: well, every merchant has interesting stuff pass in front of them, but some stuff is reserved for only the best or obviously special customers.
2) someone who specializes in the sales of magic items. This may either be a highly specialized store- potions only, weapons & armor only, etc.- or a bit of a generalist, but the proprietor will be knowledgeable as to his stock. His stock will nevertheless be limited- no point using the books as a shopping list- he has what he has, not everything contemplated within the game. If the proprietor is also the one who actually creates the items, there may be the possibility of a custom order. That takes time, though.
Rarity: depends on what he's selling. Potions and similar stuff may be available in even in the rurual areas from the local witch. Bigger cities will support a couple of shops that routinely deal in weapons & armor new-made and/or scavenged from battlefields or tombs- no smaller merchant could afford to do so. Anything of serious power would only be found in the largest of cities, and there may be only 1-2 dealers in such goods in a given country. Nobody openly trades in artifacts, and those who do sell them number in a handful per continent...or world.
In either case, availability is most likeky going to be higher than that of a modern international arms dealer. But even with that model in mind, realize that in any city over a few hundred thousand in the USA, you can find someone willing to sell you a LAW, RPG launcher or bazooka for cash, food stamps and other "currency." No, you won't get to test it, but you can definitely get your hands on illicit military-grade weapons.**
* I
do have an Econ degree
** check your local police department reports about weapons seized from intercepted shipments, drug deals, and other raids.
