Were I to introduce the bolded to my setting, it would be nanoseconds at the longest before players started having their characters replace those dealers and do exactly this as a moneymaking scheme; and I just ain't interested in DMing that. So, magic pricing is pretty much the same everywhere - if a +1 longsword costs 1800 here, it's gonna cost 1800 almost everywhere else you look for it.
I've never had PC's that either had the status as reputable merchants or the status as reputable buyers to participate in the market for magic items. Generally speaking, PCs find their magic items or are gifted to them by patrons in return for their service.
If I had players really want to get in on the market for selling magic items, they certainly could make that the focus of play if they wanted to, but they might be better off not being fighters, wizards, and that sort of thing if their goal was to be merchant princes.
As far as prices go, the standard prices are the prices the PC's can expect to sell an item at if they lack contacts and a reputation as reputable dealers. They are not the prices that PC's can expect to buy at. You can't buy, but if you could there be no expectation that sellers sell goods at exactly the same prices that they buy them at.
I should note, I suppose, that one assumption I universally make is that there are many other adventurers in the setting; the PCs are not the only ones. And sometimes those other adventurers come back to town with excess magic...
Adventurers are so not a thing in my setting that if you were to tell an NPC you were an adventurer, they would understand the term to mean "wealthy tourist without a lot of common sense". They would not understand it to mean "heroic mercenary troubleshooter".
"Adventuring parties" in the sense "for hire heroes" as D&D traditionally uses the term are vanishingly rare. It's presumed there are others out there, but they are rare enough any two of them probably will never meet. Often the PCs are the first such group in an area in generations. They may hear the tale of legendary heroes or villains that did something in the past, but they are unlikely to just find other groups of mercenaries out there. The closest to a traditional adventuring party that the PC's are ever likely to interact with is if the PC's make powerful enemies, those enemies are likely to task a group of vassals to hunt down the PC's. There are also undead slayers or other monster slayers out there for hire, but they work more like 'Witchers' in that setting than they do like D&D adventuring parties - one or two central persons plus their retainers and apprentices as opposed to a large group of diverse peers.
(In fact, a group of highly talented young people of different backgrounds working together is so rare and so momentous, that almost as soon as that group develops a reputation, rumors start flying about the gods playing with destiny, strange fates, and saints being among us.)
In any event, magic dug up out of dungeons or something doesn't happen a lot. This is a necessity of the setting. If adventuring parties are active and numerous, then no dungeon should be unexplored and the idea of dungeons that haven't been disturbed for decades or even centuries makes absolutely no sense. If there are great deeds to be done in an area, it's almost a certainty that the PC's are the only ones available to do them. There will never be a situation like 'The Village of Homlet' where there is a deed to be done, and an entire town full of NPCs with more ability, more motive, and more opportunity to do the deed than the PC's.
While at the table it very much comes across looking like a Magic Mart - when someone asks me what magic is currently on the market in town I get my spreadsheet to randomly generate a "shopping list" of what happens to be out there at the moment - in the fiction that translates to the sum total of what the PCs hear about through hunting around, talking to contacts and guilds, checking on what other adventurers have, and so forth; all taking several days or longer. The shopping list at the table might have 20 items on it but those items are never all in the same place, and are likely being sold by ten different people or groups.
Aside from low level potions and scrolls - which are available from temples, alchemists, and hedge mages in everything big enough to be called a town - the total of items would be exactly zero pretty much anywhere in my setting. There just wouldn't be things available to buy. And even if someone were selling, it wouldn't be to the PC's unless the PC's were already well known to that person. Buying a magic item is like trying to buy something that is a cross between a Van Gogh painting and an F-16 fighter. They aren't up for sell very often, and not just anyone can do it. I mean imagine going into Middle Earth and going shopping for a dwarven mithril coat, Durin's axe, the ring of Barahir, and an Elven Ring of Power. It just doesn't happen.
For that matter, I can't remember the last time a PC tried to sell a magic item. Even if you find an upgrade to your sword +1, it's vastly better to hang on to that weapon to give to a retainer or hang on to as a gift for a powerful noble to whom you need to make a good impression.
Magic items are just not fungible assets. As soon as you make them fungible assets, they lose all sense of being magical.