Exactly how big are these places in your campaigns ? For me, if there are 20 people at a given time, it's a good day, so you will at best, in large cities, find 1-2 persons who have magic items, so the probability that they have anything to trade and which would be of interest to you is extremely small.
Depends on the city. In one large city in my setting where a number of PCs have quasi-based themselves the mercenaries' guild (including gladiators, it's a thing there) has several hundred members plus a constant stream of visitors and transients. Most of the several hundred members aren't adventurers but the visitors and transients almost always are; and many of the non-adventurers still have a level or two of Fighter to them, built up over the years of doing what they do.
All of that seems really nice and still fairly low level, how does this foster magic item trade? I agree that at higher levels, our adventuring groups have usually many contacts, some with powerful organisations, some of them high level, but somehow magic item trade never comes on the table. Items are private, have a history of being found and used, and not easily parted with.
Moreover, another factor is that in 5e, a lot of items are usable by a lot of people in the party, because there are almost no generic "plusses" items. There might be a few magic weapons, but for example in Avernus, the party still has a use for those that have been found.
That's another thing 1e tends to do: quite a few items are restricted as to who can use them, most often by class but sometimes by species or a stat requirement. Further, in 1e a Fighter is proficient only with a small number of weapons chosen by the player (something I very much like), meaning there's going to be times when the magic weapon you've just found ain't gonna do you much good.
Temples probably, Guilds I'm not so sure, and who maintains defenses that are strong enough to deter theft, even admitting that trading item is something they do. I rather imagine temples in particular, but also guilds, hoading items secretely for their champions and not advertising that they have a huge wealth in items ready to be stolen.
Trusted temples and mage guilds might also double as the local equivalent to banks, meaning their security had better be pretty airtight.
As long as having arcanist create items, I'd rather them not be artificers, it's a PC class, and I don't need that for my NPCs who could create items.
Yeah, I'm distinguishing between small-a artificer (a person who makes magic items) and capital-A Artificer (the class). I probably haven't been clear enough on this; it's just that artificer is a good term to mean "person who makes and-or enchants magic items".
We dropped that AD&D thing a while ago, and although some items are lost, I think everyone likes 5e where your treasured few items are not being destroyed every time you miss a save.
I kinda prefer the easy come, easy go idea - that way, you're always getting new toys!
As mentioned, that kind of thing is sort of OK, it's just tightly controlled, and in most campaigns, adventurers don't have time to wait a few months for commissions anyway.
That not having time is what makes commissioning an item both a serious choice and a bit of a risk. Even then, now and then in my game someone will do it.
As artificers did not exist, I don't think it was the case. 3e did a huge try of assigning monetary value to specific properties, and it sort of worked, and it was linked to the creation cost, but it was the other way around for what you described, it was usability, translated into price, then into creation price.
Both the 3e and 4e strict-formulaic approaches overlook one very important thing: the item's actual usefulness in the field. In 3e, for example, a Wand of Cure Light Wounds is hella more useful than a Wand of Grease yet as they each replicate a 1st-level spell the formula sets their costs as the same
1e's pricing at least kinda looks at the field-usefulness piece but it's not perfect, and some famous typos in the DMG don't help either.
Downtime is usually boring too, you know, it's "down".
Oh, not for me! Downtime is when I get to have my PCs interact with the setting on their terms: making contacts, building structures, exerting some of that influence gained through adventuring, buying and selling items, researching spells (if of the right class), living high off the hog, etc., etc.
