I don't understand; why does it have to be all or nothing? Why does it always seem to be buying magic items vs. finding magic items?
Couldn't there be a market for some levels of magic while others just are too rare to have a market?
I think that's a good question.
To answer that specifically, I would not assume it is all or nothing.
It seems reasonable that a formal market would be more likely to exist in cheap, common magic or faux-magic items. By faux, I don't mean people lying that this here cheap cloak is really a Cloak of Protection. More like this 2sp gem will keep evil spirits away. A lower order of supposed magic that the D&D rules don't cover, but can't be proven to actually do anything (as can be said of the 2 stores within 10 minutes of my house that do indeed sell such materials for comparable purpose).
The selling of rare, very expensive items is not impossible, though it is most likely more difficult. The term "market" in economics doesn't mean store front, or anything like that. It merely represents the playing space that a person who has something to sell is trying to get paired up with somebody who wants it. So the fact that the rare magic item exists, and some people want it, means a "market" exists, though that does not mean there is a formal support structure in place to facilitate the sale of those goods. But like 2 teens trapped on a desert island with nary a sex ed class between them, they will find out where babies come from.
Rarer items are probably more expensive and more expensive items are probably harder to sell (between security concerns, legality, etc). That won't make it impossible, just very less common.
As a GM, I might raise some barriers to make it very uncommon and difficult. But I acknowledge it could happen. Especially if the players dig into it. there are all sorts of things that are bought and sold in this wide world that you and I got no clue even exists.
I would also examine percieved rarity versus actual rarity. Once you start cranking out random treasures for all the encounters the PCs have, a Cloak of Protection +1 may appear more often than you think. From a matter of statistics, there's more of everything out there than what the PCs find. The immediate playing area may seem low magic, but that could change as the scope and power of the party increases.