Thanks for the insights!
The musing as to fictional reinforcements was just a matter of curiosity; D&D influences come from all over, but fantasy fiction has been especially notable.
Buying magic items is itself not "new-ish", going back I think to Arneson's prototypical Blackmoor campaign. One of my 1970s-80s campaigns featured a branch of the Multiversal Trading Co.. The (slight) difference is the shift in some quarters to giving purchase more importance, and away from even the traditional adventure-fiction modes of commerce in such things. I get the sense that items are treated less as extraordinary magical relics and more as ordinary technological consumer goods.
What I find most striking is the reduction of enchantments to such a commonplace that players jot them down after looking in a book, as casually as picking up more arrows, lantern oil or hard tack. (More casually, perhaps, than many non-magical goods would be treated in most campaigns I've known. DMs usually wanted at least some "by your leave" rather than assuming instant availability of everything everywhere for the same prices.)
So, my personally informed answer to the titular question is, "No; it has not been 'standard DM behavior'." It might not be that in 4e, either -- as "standard" is in any case an awkward adjective for D&D -- but perhaps it is much more common. I see a step along the way in the relative ease of making magic items in 3e, and in the growth then (and maybe with 2e before) of treating "official" books as definitive catalogs of game elements (especially in the sense of players being expected routinely to browse through and pick from them, even unto magic item and monster listings).
So, gold is substituted for magic items. If even the gold appears "ex nihilo" rather than being picked up in the course of adventures, then that's yet another shift. But why the preference for mundane metal over marvelous magic?
The mentions of "wish lists" and "empowering the players" are suggestive. It looks as if the functional role of magic items has been transformed into "build components" like parts of starships in Starfleet Battles or cars in Car Wars. Gold pieces are simply the units of the "points system" used for that aspect of construction.
That's basically Champions rehashed, a trend I noticed in 3e, so on reflection not really so surprising (if accurate in the first place).