Haven't waded through the whole thread yet, but...
Emirikol said:
Do Magic Item "Shops" wreck the spirit of D&D?
Define "magic shop". I've seen everything ranging from Thingizzard's Hut (hermit in the woods) to a Thayvian Enclave ("Wal-Magic"). Each has their own in game reasons for existence. Personally, I find that having the player opening a book and saying "I want this" to be a bit unrealistic. I have no problem what so ever with a player buying a case of CLW potions from the temple of Avanthe, three sunrods and a shortsword of alchemical steel from the natural philosopher and a lucky charm (+1 to saves) from the astrologer down the street.
So, I guess it matters how you define "magic shop".
Celebrim said:
No, sure, the occasional item might come on the market - stolen goods to hot to retain, an item discovered which the discoverer has no use for, the occasional item from someone who had to part with it, or who died without heirs - but these items are snatched up quickly by the power brokers, and unless the PC's are themselves power brokers with good contacts, they'd never be a part of this. A market in magic items would be much closer to a market for weapons technology and national security secrets than a corner store, and its operation would be invisible and inaccessible on average.
But, once characters reach appropriate levels they become part of that culture and that economy. There have always been multiple economies where there is civilization. There are items for sale today for outrageous amount of money that you or I would never even consider buying. Million dollar jewelry*, for example. Once you reach "name level" you would certainly be considered a "power broker", wouldn't you? Spend a feat or two on leadership and you can have, potentially, your own private army. A bard, paladin, cleric or anyone else with a deceint charisma and Gather Information should be able to put you "in the know". Or have all of those nobles and their daughters you have rescued completely ungrateful? Surely they can put you in touch with the right people. It makes sense to me that there would be a limited supply of powerful, exotic items that are privately sold through exclusive channels. For me, that means rolling up 5-8 items, half of which are magical weapons (it's a dangerous world). I customize them a bit so that I think they're cool, and there you go.
Even here, however, I would employ some limits. For example, +3 keen adamantine longsword. I consider weapons in the +8 range and items in the 100,000 range not for sale ant any price simply for the fact that any item priced like that is too useful to too many people to be sold. But, if the players discover who has it they may be able to barter a deal.
Also, at this level of economy you have to consider what you are exchanging. The aforementioned sword is not for sale just for a collection of gold coins. There would have to be other items on the table, large gems, rare materials (ivory, ambergris) or other such things. Any noble worth their title has a mine or other resources. It needs to be something not so "easily" available like gold.
Anything that exists that is useful, pretty or confers status will be desired by people. There will arise other people who will broker those things in return for some kind of compensation. If there is a meaningful civilization there will be enough societal penalties to prevent adventurers from ransacking the town. At worst, other adventurers will be called upon to put down these villians like the rabid dogs they are. And, if there aren't any, well, there is a reason evil overlords come to power.
* =
http://www.forbes.com/lifestyle/2004/02/10/cx_ns_0211guideslide_12.html?thisSpeed=6000
Hmm, being a bit dim with formatting my linkage.