Not that this is the focus of this thread, but I can think of lots of reasons that magic item shops aren't realistic.
- Lack of information/communication. You inherited a longsword +1 from your dad, but how do you get it to someone who wants it? How do you get its worth? There's no ebay, the roads are dangerous, and chances are you live in a small village. Matching buyers with sellers becomes a huge stumbling block, in the same way that before ebay it was almost impossible to find quirky rare old items that you wanted to buy.
- Slow turnover. A shop keeper needs a relatively fast item turnover to make money, or needs to pad her margins by a large amount to compensate for slow turnover. Unless you're in a huge city or a place rife with adventurers and easy money, like a boom town, items are going to sit for a long time until they're sold.
- No immediate payoff. No shop owner wants to pay lots of gold up front, then hope an item will sell. A more likely approach are magic item brokers, people who match up buyers and sellers and take a commission.
- Theft. Open up a magic item shop, and you have every thief and poor adventurer panting at your locked-up door. That means you need to pay for security, and GOOD security. Insurance doesn't exist.
Huh. This litany has convinced me to add magic item brokers to my big cities, sly people in the know who match up their customers. No inventory problems, no theft problems, and it solves the problem of matching buyer and seller. Also, it'll take a day to a month to get any given item once it's ordered. Works for me.
This same argument would hold for any high value item in any world, campaign, genre, etc.
No Ferrari's in the real world because of "slow turnover", "no immediate payoff", "theft".
Sorry, but economics does not work that way. Your "communications" point is viable, but in such a world (like in our own world 600 years ago), the people who own the items tend to be those in the know, not farm hands inheriting from their father. Merchants, the clergy, and nobility would be the ones who actually owned valuable items and those people would also be somewhat experienced in knowing their estimated worth.
The idea of a magic item broker makes sense, but I could still see a black market of street gangs, especially for weapons.
The idea of an actual magic shop does make sense, but only in the most high end and protected section of a large town or city. The street gangs do not even walk those streets due to the number of city guards in that vicinity. There might be smaller magic item shops that deal more in components, minor potions, and scrolls, but those people would probably pay off local guards/gangs to not bother them, and/or even be backed by local gangs "Fred is really the front man for this shop, but the Black Hand really owns it. Nobody messes with Fred because nobody would be stupid enough to mess with the Black Hand.".
Btw, if one uses a 1 GP = $100 scale
as illustrated here, then one realizes that a shopkeeper could sell one potion of healing a week and be very profitable. The problem is that he cannot sell those potions to just anyone because normal commoners could not afford it ($5000 a pop?). But, there would be nobles and rich merchants and such who could afford them. Yes, having more guards on the caravan is cheaper than buying potions of healing, but any cautious merchant would have both, just in case. And if his caravan got attacked, it's likely that he would use such potions (and then have to resupply) on himself and/or best/most trusted employees (probably not on the hireling guards unless absolutely necessary).
Selling a single +1 sword makes a shopkeeper's profit for an entire year or more. So sure, such stores would exist just due to the possible profit, but they might be few and far between. And, they would rarely have exactly what a PC wants, it would take time and effort to acquire such items.
