If 10% - or even 5% - of the people at these places are adventurers with any field experience at all (i.e. not 0 xp raw 1st-level types) then you're going to meet enough of them that there's a likelihood of at least someone looking to buy or sell (or both!) some sort of item or other.
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.
In my game it's no big deal to meet other true adventuring groups or individuals either on the road or in town, and in town it's extremely common to meet non-adventuring people with class levels if you know where to find them, usually in the temples and guilds etc. noted previously.
We just don't seem to meet them often, I guess it's all down to campaign preferences.
Last session I ran the party were inteacting with a small group of Thieves who were initially the party's enemies but have since come to be, if not friends, at least neutral enough to let the PCs overnight at their remotely-located hideout a couple of times (and not even steal from them either!). En route to the swamp said hideout is in they met a small band of (probably lowish-level, the PCs weren't sure and didn't inquire too deeply) adventurers on the road, travelling from one town to another for reasons unknown.
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.
If you've never read Nicolas Eames'
Kings of the Wyld, give it a look. Adventuring parties in that setting are treated like rock bands.
I'll try to have a look, although my reading queue is quite full.
Most guilds and temples worth their salt have some pretty solid defenses against attack or theft just to protect their own goods, never mind any magic items being stored or sold through there.
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.
Where while I don't have Artificer as a capital-C Class I do have it as a profession some non-adventuirng arcanists branch into. (if a PC wizard-type wants to become an artificer, no problem: just retire from adventuring, put in several years of training, and you're good to rock)
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.
Ah - that's another difference: I have magic items being a bit more breakable than that, particularly when hit with big A-of-E damage effects.
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.
It's hard to optimize when everything on the shopping list is randomly generated (maybe with a push toward one thing or another depending on the situation e.g. if a major war has just ended there might be a few more weapons and armour on the market than usual, that sort of thing). That said, if a character wants a specific item and is willing to wait quite a while for it they can always commission an artificer to build it. The problem here is that the character's companions will 99+% certainly want to keep adventuring during the construction time, which can be months or even a year or two, meaning you either retire or risk being dead by the time your item is finished*.
* - yet another reason for items hitting the open market: unclaimed commissions!
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.
Many of the non-hereditary nobles in my setting are retired adventurers.
Same for me, but it's still only the non-hereditary...
Easy: if you went to an artificer and commissioned an identical item, what would that artificer charge you. There's your base value; and I've always assumed that to be the basis for the price lists in earlier editions.
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.
They may not necessarily prefer thise things in the moments they needs doing, but they do in general prefer the sense of realism they represent, as do I.
Realism is very relative. Playing with a higher level of abstraction is not less realistic, this is what I do at my job every day. And when I'm level 12 with thousands of gold, paying a few coppers for a meal might be realistic, but it's also extremely boring compared to having adventures, which are very unrealistic anyway. They can show a lot of verisimilitude to the genre, though, where you don't see Rand al'Thor worrying about the inventory of his pockets except in significant story-defining moments...
The training itself can be done in the background but the costs can't be waved away so easily, and IMO neither can the in-game time.
That, and downtime is important for numerous other reasons; and one very nice side effect of making characters train into levels is that it forces parties to get out of the field now and then and take some downtime.
Downtime is usually boring too, you know, it's "down".
