No. If all of the artisans were located in one place (such as Waterdeep in FR), then you could make that claim. If the artisans within a given location knew how to make nearly any magic item, then you could make that claim.
Likewise, if your game has a shop that sells some scrolls, and a church that sells some potions, but you have to travel 100 miles to get a magic warhammer, which wasn't already made for you, and which you couldn't be certain the artisan you sought would make for you, then I wouldn't claim that this was an example of a MagicMart in your game, either.
So it has to be a single discreet location in the campaign?
Does it count if you just abstract it all by saying "You go into town and can re-supply your magic items," assuming that you go to a temple for your Cure Light Wounds potions and an Akademy for your Magic Missile wands and a dwarven priest-smith for your +1 sword (assuming the town can support it, of course)? Because there's not one location in the game, but there is in the playing experience, in this case.
The core rules, at least, suggest that this is more the case. Even a setting as rich in magitek as Eberron doesn't have a single warehouse, they have competing families and differing town sizes (most of which don't have high level NPC's).
What about something like Sigil or other grand planar metropoli where critters like Mercane/Arcane hang out (effectively, almost without a GP limit)? They have a Market Ward where you can get almost anything, but they don't always keep it on-hand, and where it's not just one salesman...is that a MagicMart if abstracted, but not if drawn out?
Well, then, I suppose Q's definition ought to be given. Until he defines what he means specifically, we can only go by our own definitions.
Well, if I were to use the mighty power of deductive reasoning, I'd say that Q is implying that giant magical warehouses of magic items from Apparatus of Kwalish to Zombie Powder are aberrant, and so those who complain about them should at least be more accurate, because the term "Magical WalMart" meaningless, if what they are railing against only appears in campaigns that are actually out of the norm anyway.
This would be consistent with his argument, while a definition such as yours (that seems to make MagicMarts relatively common, if all the above examples apply) isn't. It would also be consistent with several posters who claim that "MagicMarts" were used for humor value or for tongue-in-cheek campaigns, but who nevertheless might have mundane places where people can obtain magic items in their own campaigns. Even if it is abstracted to such a level where the DM simply says: "The GP limit of the town is X, you can buy any magic item below X GP, sell stuff up to Y GP. Go change your character sheets, I'm going to use the restroom," it doesn't necessarily imply one physical warehouse location in the setting, I think.