Do you use shops that specialize in magic in your campaigns?
For me, I always seem them as one of the things that was wrong with the Forgotten Realms, especially in later editions of "the game." If magic is supposed to be rare and special, magic shops turn that on end and become a sort of magical "Walmart". I dislike such greatly.
I'm not saying magic should never be for sale, or else you'd have no way to remove excess magic (or excess funds) from the hands of your players, but I see it more along the specialist spectrum of buyers and sellers.
- Mages would probably specialize in scrolls with the occasional bauble and ever rarer permanent magic item. Maybe someone offered a Broach of Protection From Magic Missiles for that Fireball spell they had a hankering for.
- Apothecaries would sell potions, especially ones that aren't listed in the rules. These are the guys looking for rare ingredients which in turn sends the PCs all over the world looking to make some scratch.
- Specialized Pawn Brokers. Someone with enough firepower to not only have the money to purchase magic items from the PCs, but the ability to store that cash and magic away from thieves and other's that would do him harm. I see these as more of "the shop comes to you" with no set location, or at least none that is popularly known.
Remember, these folks will buy low and sell high. Probably giving 50% of book value and selling at 150% or more. It really is a sellers market (if doing trade, I'd change those numbers to 75-90% and 110-125%, especially if the NPC is really interested in what the players have to trade.)
So, how do you handle magic buyers and sellers in your campaign?