Shops plus
I'm running a Classic D&D campaign in the Grand Duchy of Karameikos using 3.5 Edition rules.
Essentially the presence of such trading entities allows me to handwave to facilitate player's fun ("I want some spell writing materials to transfer those new spell scrolls we found in the dungeon to my spellbooks") or to create adventure hooks/roleplaying scenes ("You want to buy a what?")
ORGANISED MAGIC
There is an active and dynamic market in magic items across the continent. Much of this is controlled by the state, wizard guilds or organised churches (particularly the Church of Boccob) however in the Grand Duchy an independent player has arisen: Mordakai's Magnificent Magic Mart. In the Grand Duchy, Mordakai's Magnificent Magic Mart holds a Ducal Warrant for Trade in All Magic and the directors represent the Duchy on magical trade matters.
Competition includes any Church temple of Boccob, wizard guilds, and independent specialists. Because of tax reasons, there is a thriving Mordakai's Magnificent Magic Mart in Krakatos.
Mordakai's Magnificent Magic Mart resembles a cross between an art gallery (displaying their own stock, other stock sold on behalf of other wizards), an art supply office (get your magic item creation supplies here!), a post office (in conjunction with the Wayfarer's Office if present) and Amazon (place your magical supplies order, and receive a friendly courier delivery right to your door!). [Note the latter is magical supplies, not magic items, in fact, I am using this as an adventure hook right this very moment!]