"Magic Shoppes" are definitely old school, and they make a great deal of logical sense.....although the places in which such things are sold won't necessarily be overtly called "magic shoppes". The point here, is to ignore the grognard whiners who try to engage in historical revisionism and pretend that old school D&D doesn't have institutions for buying, selling, and identifying magic items. They're wrong.
Take note that most magic items in AD&D (for example) have a "Gold Piece Sale Value", which LOGICALLY implies that magic items are BOUGHT and SOLD. And taking human nature into account, a market for magic items will be created.....since if you read old school adventure modules, you'll find them stuffed to the gills with tons of magic items.
Magic items are NOT rare in "default" old school D&D. What you do in your own campaign, is of course....your business, and your choice. Now the question is.....how do you buy and sell this stuff? If I were to guess:
(1.) Sages: A sage with expertise in the right fields of study will probably know if an item is magical, and what it does. At least, this would be the case in AD&D.
(2.) Wizards' Guilds: They can absolutely identify almost anything that isn't an artifact, but they'll usually charge you for it. You'd probably need to have a cordial relationship with a Wizards' Guild, for this to happen though. If so, it will probably be pretty easy to sell magic items to them. Whether or not you get a good price is another question entirely.
(3.) Alchemists: They can probably identify most potions. It's safe to say that they sell magic potions.
(4.) Merchants & Auction Houses: These sell magic items, and have their abilities pre-identified and pre-catalogued. There will be reputable auction houses and magic shoppes that sell magic items, and other "shady" auction houses....that you cannot fully trust. The magic shoppes might not actually be called "Magic Shoppes"; they might be called something else. Take note that many places will not sell magic items openly, due to some places having strong social prohibitions against wizardry. In other words, you're probably not gonna find a "magic shoppe" in your typical farming village....so you'll have to go somewhere else to buy or sell magic items.
Another thing is that not every magic item you want is going to be available when you want to buy it. In fact, most magic items that you might want at any one moment probably won't have instant availability. Many magic items will be nearly impossible to buy, like say.....a Ring of Three Wishes. On the other hand, getting hold of most magic potions and oils should be hilariously easy.....if you have money. Likewise, getting your hands on weapons and armor BELOW a +3 should be pretty easy, considering how common they are in adventure modules.....provided that you can afford it. After that, magic item availability gets kinda murky. Wizards' Guilds will probably be very discriminating in regards to whom they sell wands, staves, and scrolls to. And you obviously won't just find a Ring of Three Wishes just sitting there in some rinky-dink magic shoppe somewhere.
If I were to guess, I'd say the "Gold Piece Sale Value" in the DMG is a guideline. You'll occasionally pay less, but you'll often pay more. Some vendors will permit haggling, while others won't. If you want to roleplay this stuff, go for it. If not, you can just do this stuff "off-camera". Some people enjoy this stuff, while others don't. Just remember that the people who buy magic items from adventurers probably have both multiple contacts and MONEY. As always, YMMV.