I personally cannot require anything... And you personally need to relax.
What I am suggesting is that
a) Though slightly more glamorous than farming, I suspect that there will be few PCs acting as full time item creators. I suppose people with a lot of free time could make background PCs for that sort of purpose, but then the "XP gained by killing stuff" problem arises. In short, PCs are not going to account for the realistic selection of item brokers a place like the red dragon inn would be frequented by. Ergo, capitalism ain't gonna work.
b) The prices set in the DMG are presumedly based on game ballance (as are the prices for more mundane items I imagine) and it would be a mistake IMHO to let new prices be set without the average PC being aware of the "fair market value" of an item (with craft, proffession or apraise checks as always to prevent being ripped off).
c) Raw capitalism has never struck me as a required part of a D&D universe, and certainly not one like Living enworld where prices for ads are set on Joe's say so. Lacking the imaginary hand of the free market, I would be suprized if certain high powered NPCs didn't step in with some friendly advice to those who think they can charge outside of the accepted range of prices for items.
Kahuna burger