Say the PCs come back from the dungeon with some loot to spend. One PC wants to buy a suit of full plate armor (1,500gp), one wants to buy a spyglass (1,000gp), one wants to buy four tanglefoot bags (200gp), one wants to buy a heavy warhorse (400gp), one wants to buy a potion of cure light wounds and a potion of invisibility (50gp and 300gp), and one wants to buy a +1 chain shirt (1,250gp). Are any of these purchases made at any kind of Walmart store? Are the magic item purchases more game breaking than the mundane item purchases?
To answer, IMHO, it depends. Are they in a big city? Are Heavy Warhorses common in the area? Also, Full plate, despite being expensive, does not require the expendature of xp, and its a craft skill, so not so far out of reach as spellcasting--time seems to be the largest investment here. XP doesn't grow on trees, despite some adventurer's feelings to the contrary. And once you get beyond a certain point, it seems like spellcasters must pooping magic to keep up with the apparent availability levels. And not every town/village/mudhole is going to have everything someone needs, even if it falls within the max item cost. Why? A little math and some common sense. Geography, industry, and politics can also play a part. In any but the largest cities, how many high level wizards or clerics exist, per the current rules in the DMG?
Say you even have a Large City. Pop about 18,000. In that city, at best you have three 13th level wizards, six 6th level wizards, and 12 3rd level wizards, 3 13th level sorcerers, six 6th level sorcerers, 3 15th level clerics, 6 7th level clerics, 12 3rd level clerics. The same for druids. So at best you MIGHT see a fair availability of mid level items ASSUMING some of those 6th and 7th level folks spend their time selling or making items as well. How many of those dozen high level characters actually make stuff? Realistically? How many clerics would even sell items to those outside their faith? Even if you say half of these guys have the ability to make items, and half of those actually make them for general sale (which I think is being generous) you got about 3 high level casters churning out magic . . . . .do the math.
Trade with other cities? Sure, but caravans can get raided. Detect Magic is easy to come by, so it would be a costly endeavor to effectively guard such trade, making it . . . not so much a money maker as some would think. Items made in the past and unearthed by adventurers? Sure, but how many adventurers do you got floating around? Again, take into account the numbers and levels generated by even a fairly large city, count the number of cities you may have of a notable size in your world, and again, it becomes a very niche industry . . . .unless your world is chock full o' Metropolises . . . but then that would seem to strain credulity a bit. And even so, you'd find the number of folks available to be less than what you would need to reasonably allow EVERY item someone wants, within the village/city/town's GP limit, to be available. Again, unless every caster that CAN make items DOES, and every character of a PC class adventures, and every priestly organization, or arcane organization for that matter, just sells their stuff willy nilly. Sure, some would, that's reasonable, but not all. Not all casters are going to see the GP as the be all end all to power, and prefer to keep their shiny stuff. Heck, especially since even the moderate powered stuff can start to drain their xp pretty rapidly . . . not to mention creation time (which would start to hedge out that much needed adventure time to recoup their xp loss)
So, again, IMO, the whole "Magic mart thing" is the idea that anything that can be available, therefore must be, despite even the basic application of common sense, or any attempt to form some sort of pseudo-real economic and political structure that's even good enough to allow one to suspend disbelief. I'm not a hard one to go along with the whole suspension of disbelief thing, but even I find the default assumptions of magic availability to be stretching it towards inplausability. It works if you assume your NPCs have no personalities or agenda of their own, other than to trade and create magic for your characters to purchase, but I think the system falls short and indeed looks Magic Mart like.
Anyway, that's my take.
