It depends. Basically, I like the 5e stated premise about magic items being rare and special, but I try to extrapolate what makes sense into the world also. So the fine arts market makes sense, but that doesn't just mean auctions--there may be some shops based on locale.
In any place where it is assumed there are wizards, whether resident as more that one or two, or traveling through, there will probably be a magic shop.
A magic shop is essentially a spellcaster supply shop. They sell spell components (I assume PCs go to such a place in down time to fill their spell components pouch with cheap consumables), spell book and scroll scribing inks (which they have to specifically state they are buying and keep track of other than the freebies a wizard gets on level up), arcane spellcasting components, blank spell books, etc. Sometimes they will offer the service of allowing you to learn a spell by copying it from a spell book they have. In that case, I randomly roll to see if the particular spell the characters are looking for is on offer there (unless it's something that should definitely be or not be available).
In some locations, you may be able to pay an individual wizard for such goods and services rather than a shop. It isn't considered odd to ask the local wizard when a shop isn't present.
As far as actual magic items, potions of healing are probably available somewhere in town--usually at temples. A random selection of spell scrolls are often available at a magic shop, and can often be commissioned at shops or temples (again, it's always a random selection, not the whole list). Potions are the same.
Beyond that I will tend to roll some additional random items from the DMG tables, based on the size and commonality of magic in the locale. The number might be zero in some places, but likely is 1d4 in a small-medium shop, which is almost always the biggest shop available. In Waterdeep, there are a few such shops, some of them specialized.
Now, in a real planar metropolis like Sigil, you will have full-fledged magic item stores, and even potentially magic items in high-end version of other stores, like the party found a bag of holding in a clothing store. In those full-fledged magic item stores, they will have some specific randomly rolled items available, and probably a (tiny) percentage chance of having any particular other magic item the party might specifically ask about, or being able to get one for an extra cost.
One can never just assume what you are looking for is available. When our party was in Sigil they wanted to get some bags of holding. I think they found one at a clothing shop, another couple at a magic item shop, and the fourth may have been at yet another magic item shop. Each shop has its own interesting character and NPC proprietor.
I should also mention that it's often a better deal to trade magic items they have at the shops rather than paying in cash. If the shop is interested in the item they will get a better deal, though if they aren't it may be a worse deal.
Other than potions and scrolls, I don't actually treat common rarity magic items as common. They tend to be luxury baubles that only make sense existing in areas where magic items are widespread like Eberron, or to a lesser extent Sigil. While a PC with Arcana has probably heard of most of the items in the DMG, they probably haven't heard of things like XGtE "common" items unless it's Eberron.
To me, this keeps magic items in a place that makes sense in the world. Players can relatively easily get supplies they should be able to, but finding any specific item requires luck or work and can't be done just anywhere.