Sorry what?
Let's not make this more difficult that it already is.
We're talking about the magic items in the DMG. No changes.
Then you're going to end up with the "Big Six" problem again.
There are items in 5e (like in 3e) that are obviously better than others, even if you set the prices far more narrowly (like 3e/4e did). As long as you have items that directly impact "the numbers" (AC, HP, Attack, Damage), they will always be far more valuable than items of limited or situation value (like Potions of Longevity or a Wand of Secret Door Detection).
If you were interested in comparing the cost of say, a cloak of protection to a cloak of elvenkind, my gut reaction is to price the cloak of protection way above the cloak of elvenkind (say, 5,000 gp for the CoE, but 50,000 for the CoP). There is no way I'd price an item like Gauntlets of Ogre Power for less than a million GP.
In practice this is not a concern at all. Just because my fantasy world "should" "logically" become an Eberron in the far future doesn't mean it will.
Anyhow, there's no need to worry about that now. Or ever.
Sure, and there's no logical reason why magic items can't be bought and sold. A world where magic is a commodity eventually ends up facing the Eberron question: if magic items are just gold + time, why wouldn't you make them to better peoples lives? Once you answer that, you get a line of logic that leads to magical lightbulbs, trains, telegraphs, artillery fire, and first aid kits. You can deny it, but that's no more nonsensical than saying nobody would ever sell you their +1 sword either.