Check my sig to see why buying and selling magic items is a good thing.
Right now, I'm running Shackled City. There is a magic shop, but it has a specific inventory of around twenty items. But there are also alternatives to "the Magic Shop" that round out the experience. To start with, the church of St. Cuthburt sells low level clerical scrolls and potions.
There is also the Cathedral of Wee Jas. Magic item creation is an important duty of the church. I declared that the church would create and sell arcane and divine magic items of up to 6th level caster level at book price. Anything of higher caster level would require a favor from one of the high priests. This would take the form of a social challenge for the PCs to negotiate.
To buy a magic weapon or armor, the character needs to buy a MW weapon or armor from a weaponsmith or armorer, then take it to the church of Wee Jas to get it enhanced. In other settings, the martial churches would enhance arms and armor, or the weaponsmith would do it himself as part of the crafting of the weapon.
I have also added to Cauldron a noble clothier. The noble clothier makes the noble and royal outfits and jewelry for the upper crust of the town. They also sell enhanced clothing and jewelry. Mostly those items that would interest nobles. Clothing with illusionary highlights; appearance enhancers. Social skill buffers (such as the circlet of persuasion). Defenses against charms and compulsions, illusions and lies. Catch-22: to walk into the door, you must appear like you belong there. But they are the only clothier in town that sells noble clothing.
Gather Information, Knowledge: Local, and Knowledge: Nobility are invaluable skills to find buyers and sellers of magic items. You can find out who owns what in the area, then you can try to convince them to part with their item for cash. Just as magic items are more valuable than cash for PCs, there are NPCs on the decline that need to liquidate their assets to take care of other problems. And right now, their magic item is a white elephant. Or at least that is what the PC is going to try to persuade them.
Again, check out my sig for why being able to select magic items is the game within a game that is fun for PCs to play.