So you admit there is a market for magic items?

Even in a world where magic items are rare.
*I* actually never completely shut out the idea of magic items for sale. My concern is for
easily available, fixed price, and commonly assumed magic item acquisition via gold pieces.
Let me parse that for you. Easily available = large towns and cities have items for sale (or are a simple commission away from it). Fixed price: the DMG sets a price to the gp, and sets rules for buying/selling them (buy at full, sell at half). Commonly assumed: these rules will be followed since they are Rules As Written.
I would rather magic item sales be a.) difficult, almost a quest to themselves; b.) at a negotiable price depending on the situation; and c.) at the DM's option.
Surprisingly, the DMG rules in the downtime section do that perfectly. The problem some people are having has been that they are not through enough for my "concern" sentence to be true.
I am not sure you are proving your point. You are actually saying on the one hand you want a fixed price for what the PCs might be able to sell a magic item for, but do not want the PCs to be able to buy under the same rules. Which is fine, but one should be upfront about it. A fixed price is no more subject to abuse than a non-fixed price, especially if one considers the fixed price more of a guideline than a hard and fast price.
I didn't say I wanted a fixed price, but I'm also not stupid enough to allow King Bob to buy a +1 sword for a million gold. However, PCs are generally the ones in want so the market preys upon the needy. Capitalism baby!
And here is how fixed prices screw up the system. Lets say your PC acquires 5,000 gp. He goes to Waterdeep* (or sufficiently large city) and hits the bazaar to buy a +1 amulet of natural armor, a +1 sword, and a +1 cloak of resistance. (The three items are a few hundred over 5k, and the PC can afford it all). However, he goes to the magic sword vendor and the vendor says "Well, there's a war going on near Neverwinter, so +1 swords a 4k each".
The PC is now Angry. He knows the DM is overcharging him, and that "by the rules, he can get the amulet too" since the price for a sword is set. Furthermore, the DM is watering him down for his Wealth Per Level guide since he's effectively stealing 2k away form his proper reward.
However, if the price is a range, the PC can't call the DM for overcharging him since there is no set price. He doesn't like it, don't buy it.
As for the rest of your argument, it does not have to be an either/or sort of thing. Personally, I don't have "magic marts" in my games, but magic is for sale. There are items that are readily available and if the PC wants something not available they have to work for it in some way, order it from afar, pay for middlemen, pay for transport, etc. This allows for haggling, auctioning, con-games and the market to do its thing, while still treating magic as a commodity. Again, as soon as the players want to sell an item or service, magic is a commodity whether you as DM like it or not and you should have some idea what you are going to do about it before hand.
As I said, a few items bought and sold using the DMG guidelines and some common sense economics is enough for me. Picasso's are sold everyday; neither are magic items. What I don't want is the 3e system of expected wealth and PCs with carts of cloaks of resistances (needed to keep NPC math going) and going into town to sell for 500 gp a pop so they can add another plus to their +1 flaming, keen bastard sword. Those days are gone and good riddance!