One thing that amuses me is that the RL examples people give of things with huge markups are all "luxury" items (yes, even soda). For an adventurer, magic items are tools. Buying a top of the line sword isn't an extravagant, showy expenditure, it is an effort to stay alive. "Sane" adventurers, if such a thing existed, would be willing to spend a goodly time optimizing their gear, even if the theoretical gain/time ratio was worse than adventuring, because adventuring whilest undergeared results in death.
Adventurers are also relatively well set up for the magic item trade. Once they enter the paragon tier, they have *huge* mobility and long range communication: exactly what you need to efficiently engage in a trade with a diffuse population of clients/suppliers. It is hard to argue away the existence of an equivalent to EQ's pre-luclin EC/WoW's pre-linked bazaar IF/Org or any other "trade zone" in a MMORPG. And once you have a "trade zone" that feeds off of a continent's worth of adventurers, magic items become liquid. You won't get a brutal cartel either: repressing adventurers either results in them relocating, or them killing you.
Of course, before the paragon tier, you will need to hire teleports/do business with people with teleporting capacity. A less liquid MI trade, but still functional (which a 6-1 exchange ratio isn't).