The case is an easy one assuming the default high magic setting...
Civilizations, even primitive ones, thrive on specialization and trade. Clearly magic exists, either it is readily available to be found or made. People with money will want magic. People with the means to find or produce magic items will want money. A price will be arrived at and presto....a market will exist. If the it is a valid adventure hook for PCs with little money or some other need to be hired out to go find a magic item then it is just as valid for PCs who have money that they need less than a particular magic item to trade it.
I don't think the default assumption is a high magic setting. Especially with the change in the way magic items work in 5e.
Whether magic item shops make any sense depends on precisely how rare items are, I agree. The implication is that the default position in 5e is that magic items require exceptionally strong and skilled people months or years of their life to make and require extremely expensive and rare components. These are the kind of things that one only makes for themselves or for a king's ransom worth of money that would set someone for life.
When the component to make a Ring of Protection is a leaf that falls from the longest living tree in a particular forest filled with monsters and guarded by dragons that is freshly picked during a solar eclipse, there WILL be Rings of Protection because of how powerful they are. But they aren't going to be sold in stores all over the world.
Let's assume this is true. It's likely that there is about 1000 of them produced throughout all of history. Especially in most D&D campaigns where civilization has lasted a long, long time. 10,000 or 50,000 years worth of time where Wizards were creating magic items isn't out of the ordinary for most D&D worlds.
Given another trope within D&D is that entire empires come and go and are often forgotten. We can assume that 80% or greater of the rings created are lost somewhere in undiscovered tombs or guarded by ancient guardians since we can assume more rings were created BEFORE the current generation than during it.
It's likely 80% of the remaining 200 of them were passed down from generation to generation and are considered heirlooms and birthrights and likely wouldn't be sold at almost any price.
The last 40 or so of them are likely in the possession of adventurers. They might be willing to part with their rings, I suppose, since many of them are mercenaries. But given these magic items protect them from danger and they spend a lot of time in danger, I wouldn't count on it.
I assume magic items like +1 swords and +1 armor would be a lot more common. It's likely the process for creating it is cheaper and requires less rare components. There MIGHT be a store that sold these as the number of them would likely get to the actual level required to have an economy.