To clarify my vote: in most of my games, I want magic to be rare. So there are lots of things that the bazaar sellers SAY are magical... but really aren't. They may grant a simple +1 through good construction, but they don't have the same power as a true magic item. Same with spellbooks and other things found in dungeons. The only way to be sure is to test it out...
I think it is reasonable that magical items, like anything of value, might be bought and sold, but I don't think that any given item is likely able to be had on demand. A merchant might have some assortment of curiosities of dubious provenance; that's not the same thing as shopping off of the magic item list.
low value magic items for sale frequentyhigh value will cost you dearly
Yeah, generally potions are fine as a sale item. High-end magic could conceivably be purchased but that sort of thing should be perilous to own, period.
There was a thread on one of the RPG forums--Dragonsfoot?--that discussed this in detail. In my campaign PCs can purchase magic items, but there are no shops--go-betweens arrange appointments after contacting interested parties, and the transaction goes down more like a major deal between rival mob bosses. And like Cole says, there's no guarantee that they have exactly what you're looking for. And there are charlatans galore.
My preference is much like Jayson's, above. A nice effect of this is creating a whole range of new npc types and new organisations and networks. It would be a dangerous business, and it would have to be something they wouldn't want themselves. I would equate it to trying to buy or sell something stolen from the Louvre.In my game minor things like potions are available, but not in any great frequency. They're too expensive for the vast majority of people to buy, and therefore an alchemist couldn't support a business based on it. They would have to deal mostly in other things.
I let PCs purchase things like potions from alchemists, assuming the alchemist in question knows how to make a potion (i.e. they have to be a master, and at most they know a single formula). They can commission other items from high-level magic-users who always live in the wilderness and who demand all sorts of annoying and dangerous favors.
I'd have to concur with Matt and the rest. I'd let potions be purchased through an alchemist but I'd restrict other magic tiems from general sale. That's not to say it couldn't happen on occasion, but I don't have an Magic Item Flea Market or anything of that nature.