In my current DF game, you can just go out and buy minor items, and order special magic items. They'll take a long time to get, but it's just a matter of money and time. Pay the money, wait the time, get the item. Only NPCs can learn the Enchantment spell, to limit things further and encourage delving over making items for sale.
This perfectly suits the pickup game I run. Minor magic items become another expendable, or a class of gear better than some mundane gear. Greater magic items are great for use or for sale, because they take time to get but I have a method to convert them to cash for people who just don't need whatever it is, no matter how good it is.
But in my previous campaign, I wanted a bit more wonder to magic items. A bit more of "magic as special item" and not "enchanted per GURPS Magic, p. 16-22." A bit like JD Jarvis's ideas on how magic isn't science and that magic items aren't just up for sale.
At the same time, I liked the idea that minor items, charms, etc. were for sale. Not the least of which was because my players were making characters and I was mapping the campaign around their character concepts - and one started with magic armor and a magical power tattooed into his skin. So that had to be possible.
I did three things:
No "Item" Portions of Most Spells - Most spells in GURPS have an Item line, where it details what you can enchant and how much it'll cost. The knowledge to make these comes with the spell plus the Enchantment spell. I said, simply, most Item portions of spells had been lost. What they did have was relatively minor, or straight-up Enchantment (such as Fortify, Deflect, Staff, etc.) So you couldn't go order up a Wand of Fireball because they couldn't be made anymore.
Nasty Critical Failures - I hate being reduced to just rolling on the Critical Spell Failure table. Usually I can think of something much nastier, and more appropriate. That's for when I just can't think of anything. So critically failing led to bad things - and the more powerful the spell, the more powerful the failure.
Risky Magic - Finally, all magic used in the game was rolled for, at least in theory. I took the idea that the cap on Enchantment spell rolls was 15, which is about a 95.4% success rate. A 16 fails, a 17-18 critically fails (1.9% of attempts were critical failures). So what if all spells, all attempts to enchant things, all routine "magic me a sword +1/+1" turned out like this?
Extending a 3d bell curve roll system for adventuring skill rolls to the world as a whole is usually foolish. You end up with planes crashing by the dozens everyday, armies full of amputees after sword practice, huge amounts of patients dying in top-line medical clinics, and farmers mismanaging their crops enough to regularly cause famines. But for magic, what the hell, I liked the idea that even routine use of magic was a failure almost 5% of the time and a potential disaster 2% of the time. Sure made for reluctance to use Continual Light when a torch wouldn't potentially inflict supernatural harm on your household.
What this did was explain why you could go out and order a magic sword - but the more magic it took, the longer it took, and if it failed in the end, everything was lost. Including, potentially, more than just the time and the sword. It could be cursed, the enchanter could suffer, etc. So only the really powerful types could afford it, or compel this to be done. It was too risky, because there was this very real chance of magical failure and the more power in the spell the worse the effects of the critical failure. Few had the skills to try, and the more they tried the more the law of averages caught up with them and brought it all crashing down on their heads.
It also made multiply-enchanted magic items much more rare and worthwhile. It helped explain why there were some wholly unique items, too, because a critical success would lead to some amazing breakthrough - but just this one time. But since screwing up on a minor enchantment wasn't that bad (low power, so low consequence), you could make a living at being a careful enchanter of minor items. I wish I'd thought of JD Jarvis's explanation on astrology and this approach on different item components - they would have fit perfectly last campaign!
So that's how I ended up with off-the-shelf access to Fortify and Deflect, or Umbrella items to keep the rain off, but also kept people from going out and shopping at the Magic Item Emporium's Emporer's Day Sword Sale for a new +3/+3 Sword of Ice Weasel Slaying. Yet I left some space for unique and powerful magical items, such as the sword Grimslaughter or the rapier Malice, or this strange Buckler of Warding that did things no one could figure out how you'd enchant, and so on. It "worked" because the rules as the players understood them allowed for both rare magic, powerful magic, and minor items for sale.
Oh, and selling magic items? It happened, although more often it was trade. You just had a hard time finding someone with the cash to lay out. Trade to someone powerful for influence occurred, usually as part of a larger and more complex trade ("You get these magic stones, and our service for a year, and you get our friends resurrected, and provide us support right now to kill the guy that has this other item you want.")
It seemed to work well. But for my DF game, it's not what we want now. And two of my DF regulars played that game from start to finish, for like 10 years of play, and I didn't want to make them deal with the same problems again. So I chose another route.