In the City-State of the Invincible Overlord, the first city ever released for a RPG circa 1977, you can wander up Regal Street look left and see...
The Sorcerer's Supply House,
where various magical ingredients and items are sold.
Over at the Bard of Valiant, they are talking about how magic is too common here.
I agree that an over abundance of magic items is not the best for most campaigns. However unfortunately Bard of Valiant is wrong about it being a defining characteristics of Old School Gaming. From the beginning it was always a point of contention whether magic items should be able to be bought and sold. Or remain mysterious artifacts woven in myth and legend.
The very first city detailed for RPGs had a magic item shop. Viridistan the City-State of the World Emperor had several.
My own solution is to allow COMMON magic items to bought off the shelf. +1 Arrows, amulets that give +1 to saving throws, certain potions, and scrolls, etc. Maybe a +1 sword will find it way into the shop.
The market or high end magic items I treat similarly to how high priced art and collectible are treated today. They are sold in auctions where only select bidders are allowed. A small group of brokers scattered throughout manage these auctions. These brokers also work hand and hand with the Overlord and other rulers to insure dangerous artifacts don't fall in the wrong hands.
Magic items can be commissioned and these have to be arranged through the various magic guilds. +1 enhancements are relatively easy to commission however more sophisticated enchantment will mean showing that you have some money in the bank.
I find the plausibility of the system tend to have players accept it. Players become reasonably satisfied. They can get useful "magic tech" to aid them in their adventures, and gain a outlet for the items they acquire but do not find useful.