Altalazar said:
I don't know of anyone who does it like that. A magic shop with a handful of specific items seems not only to not break anything, it just plain makes economic sense. Just because something is "magic" doesn't suddenly make it defy all the laws of economics.
Well, let me just put on my economist hat. I suppose what some people object to is the idea that a shopkeeper could make a living just selling "magic items". This may indeed be true depending on the rarity of magic in the game. Perhaps it may be good to think of it in terms of demand and supply, and what kind of market structures may emerge under various conditions.
1. Steady demand, steady supply: For want of a better term, this is the D&D "baseline". There are spellcasters who regularly produce magic items, most likely low-powered and inexpensive items such as potions and scrolls, and there is a steady demand for such items, possibly from a fairly well-off "middle class" who can afford a few luxuries, magic-quality equipment, healing potions, etc. Magic shops can and probably should exist under such conditions. These shops may be tied to magical academies or temples, or they could simply be intermediaries between the buyers and sellers.
2. Low demand, steady supply: Most people don't really want or need magic items. On the other hand, spellcasters are more than willing to create magic items, provided the price is right. Magic shops will probably not exist under these circumstances as there would be insufficient trade to sustain them. However, magic items may still be bought and sold, but alongside their functional counterparts. A shop selling weapons may have a few magical ones for sale, for example, and a rogue's underworld contacts might sell some items that assist in stealth. Alternatively, magic items may simply be commissioned when required.
3. Steady demand, low supply: Spellcasters cannot or will not create many magic items and thus the supply is low. However, there is great interest in obtaining magic items. Again, magic shops will not exist as there is nothing to sell. The rich and powerful would probably employ agents to keep track of known items and make offers to buy as and when the owners no longer require them (or need cash more). Once in a while, an auction may be held and magic items sold to the highest bidder. However, more often than not, buying and selling is a private transaction.
4. Low demand, low supply: A market simply does not exist for magic items because nobody wants to make them and nobody wants to buy them. If you want magic items, you'll just have to find them or make them yourself.
I'm guessing most campaigns will fall into one of the above categories, most likely (3) for a low or rare magic world.