For my games collectively, it can be assumed that the locals do not have the wealth that adventurers do. With how much adventurers move around (and act in general) running a shop that serves them is not a profitable business model. Sure it is possible for a random shop in a random town to have a single magic item on display. But that weapon may likely be the "personal" weapon of the local law enforcement and or local guardian keeping the farmers safe from minor threats.

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In my games the only (guaranteed) profitable way for a local craftsman/shopkeeper to run a shop that supplies adventurers is to be backed by the various guilds.

So 90% of the items an adventurer needs (or at least in the amounts they need) are available from the auction house.

Most low end items are not actually auctioned off, but anything in mid to higher tier is sold at auction.

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The auction house is supplied by and in return supplies multiple metropolitan regions so is exempt from the usual (math that measures buying and selling capacity based on the population of the region)(see dmg for more information on settlement wealth)

The auction house has in house appraisers and the capacity to buy most things, or sell them on consignment. They buy most equipment at 60% of market retail price, or in the case of selling on consignment 75% of the auction sale of the item.

Adventures are free to craft their own items, but with supply and demand, it is hard to get your hands on crafting reagents that are not sold even before they are manufactured.