Magic items are so diverse that I don't think that a single market model will cover all the way in which they can be sold.
Some magic items are relatively inexpensive, not particularly dangerous, and most people would find them useful from time to time (or just as insurance policies), like blessed bandages (from the Magic Item Compendium), or maybe even potions of cure light wounds. For such items, there might be shops that carry them as part of their regular inventory. These shops might not excusively deal in magic items, but would probably deal in related goods, e.g. herbalists for curative items.
Other items are more expensive, or would only be useful to people with special jobs or training. These would include scrolls, wands, magic arms and armor, and minor protective gear. There would probably only be a small number of establishments dealing in these items, but those who need them can probably find them with a little effort. Again, they might not deal exclusively deal in such items, but would have related interests, e.g. a temple, a mage's guild, an arms broker who supplies equipment to the local lord, etc.
Other items are more rare and expensive, e.g. weapons or armor with special properties, and obtaining a specific example would require more work. Occasionally, one might be available from a high-end broker, or might show up in an auction house.
Other items are almost unique, and those who want them may have to negotiate directly with the owner to obtain them, or, if they are lost, find them themselves.
Lumping all magic items in the same category is like expecting to buy asprin, a high-end rifle, a vintage vehicle, a rocket launcher, and Van Gogh's Sunflowers in the same way.