Some things to consider i.e. the availability of purchasable magic.
1) Spellcasters or those benefiting from an item created by spellcasters are generally the only people capable of high speed communications and rapid travel in the D&D world.
2) Spellcasters are the only ones capable of creating the items we are talking about.
3) Spellcasters are much less dependent on magic items to achieve greater power as they go up in level than are other, non-spellcasting, classes (in other words, a 12th level Fighter with no magic items is at a significant disadvantage, compared to a 12th level spellcaster with no magic items).
Conclusion: Spellcasters monopoly on magic gives them a huge amount of power. It is
greatly to the advantage of spellcasters to organize themselves and regulate the quantity and type of magic they produce and sell. It's good to limit quantity because that keeps the price artificially high. It's good to limit the type of items they produce because they can then assure that other (non-spellcasters) never achieve parity with them in terms of power. It is good to limit the use of communication magic because that ensures the spellcasters will not only decide who knows what, but also that they know things well before others. Therefore, an organized body of casters will limit the availability of magic items and powerful spells because it is in their own best interest to do so.
Corollary: The spellcasters who benefit most from this arrangement are high level casters. They are also the most capable of enforcing the arrangement. High level casters benefit the most because they are so much less dependent upon other (non-spellcasting) classes for their own survival and protection. Lower level casters might be tempted to create powerful items for other classes. They might want to do so for their companions in the hope that said items will increase their own overall chance of survival (a good assumption). They might want to create items for others in exchange for pay in the hopes that the increased wealth will bring them comforts and increase their overall chance of survival (again, a good assumption). High level spellcasters are much less likely to care about empowering their allies (they don't need more powerful allies, they can just create more of the ones they already have) or about making money (it's likely they already have tons of phat lewt). High level casters are the only ones who can create really powerful items. Therefore, an organized cabal of high level casters will refuse to produce really powerful items for most people, will use coercion, persuasion and bribery to force less powerful casters to adopt the same policy, will actively oppose other high level casters who refuse to cooperate and will most likely work to limit the amount of "found" magic treasure that finds its way into the hands of normal folk (either by buying up anything that becomes available for sale or by simply taking it, if they are less than scrupulous).
Result: Mr. Fighter can't buy or commission the +3 sword and +4 belt of strength he wants because spellcasters like the fact that he's dependent upon them casting
Greater Magic Weapon and
Bull's Strength to get the benefits of those magics. The only readily traded/purchased magic items will likely be low-powered items usable only by spellcasters (such as scrolls, wands, etc.). Even those items will likely be limited to containing innocuous spells such as
Mage Armor,
Sanctuary,
Resist Elements and the like, because in addition to not trusting other classes, it's likely spellcasters will not trust each other greatly either. It only takes one time of selling someone a Wand of Fireballs and having him turn it on you, reducing your home/laboratory/shop to a pile of smoldering cinders to decide that maybe handing out WMDs to anyone who wants them isn't such a great idea. Even low level spells such as
Charm Person are dangerous to spellcasters in the hands of others (anyone can fail a Saving Throw) and are not likely to be given out except to very trusted colleagues or people who are already capable of doing you harm anyway (such as a more powerful spellcaster in exchange for an item you want).
Of course, when people find out that spellcasters are hoarding all the neat-o magic stuff for themselves, there will be disgruntled individuals who speak out against the Wizards and their selfish ways, but that's what spells like
Disintegrate and
Mass Suggestion are for, right!
