However, if you treat ALL prices realistically, by the time they have enough scratch to buy magic items, they will be used to the printed prices being guidelines only, and the risk of merchanticide drops significantly.
Well, yes. The PHB has a listed cost, 10 GP, for a cow or a square yard of silk. <sarcasm> Because the amount of gold you can get by selling a cow or a square yard of silk has been constant, in all locations, across all of human history.</sarcasm>
***
Terminator: The .45 long slide, with laser sighting.
Clerk: These are brand new - we just got them in. That's a good gun. Just touch the trigger, the beam comes on and you put the red dot where you want the bullet to go. You can't miss. Anything else?
Terminator: Phased plasma rifle in the 40-watt range.
Clerk: Hey, just what you see pal.
Terminator: The Uzi nine millimeter.
Clerk: You know your weapons, buddy. Any one of these is ideal for home defense. So uh, which will it be?
***
Perhaps the Terminator was angry that the phased plasma rifle was not available at the cost listed in the rulesbook?
Just as a certain participant in this thread might be angry if their PC could not buy a musket for 500 GP, the cost as listed on DMG p.268?
***
Here's another angle.
For purposes of argument (stipulating imperfection), let's divide D&D play into some rough categories:
(B) Events played out in close detail and with many die rolls, such as round-by-round combats.
(C) Events played out in close detail and with few or no die rolls, such as long discussions held entirely in-character.
(Genre example: the Council of Elrond.)
(D) Events played out with less detail, and few or no die rolls. For example, learning a language, using the downtime rules. Got 250 days and 250 GP? Add Elvish language proficiency to your list of abilities. DM determines how long it takes, might require some INT checks, but if a fellow PC knows Elvish and their player declares that they'll teach, then Bob's your uncle.
(F) Events played out using NONE of the DM's time and attention, decided solely by a player, within a standing agreement or honor system. For example, using downtime to recuperate (PHB p.187). No DM decisions are necessary. Alternatively, using GP which the PC has in their purse, to buy mundane, widely available items, such as lamp oil or re-stocking their supply of (normal) crossbow bolts. As a DM, I would rather trust players to just do that stuff on their own, when the PC is in a major city between adventures. It's not worth even a second of DM-player consultation.
(G) Puns, lewd innuendos, Shakespeare quotes (wait, that's redundant), etc., which don't affect the PCs.
I don't want buying a +1 sword, *at my table*, to happen in category (F), unless the setting is magic-rich. Buying healing potions and scrolls of Lesser Restoration MIGHT be in category (F), depending on setting and on location within setting, but if a PC buys a dozen potions of healing, they might well have bought out the entire locally available supply, which means that another PC can't also buy a dozen PoH in the same city on the same day. So it's more category (D); my answer is likely to be yes, bar scenarios such as "Your elven wizard buddy bought all my PoH yesterday, try again when I've brewed another batch".
I'll never run a game in which buying an Ollamh Harp or the Apparatus of Kwalish is category (F). "Wait, you bought the Apparatus yesterday? So did I! Well, that's redundant. Hey DM, actually I bought a Deck of Many Things instead."
I also don't want PCs to sell a Staff of Power as a category (F) action. There are rules on DMG p.130 for selling magic items, but I don't want players scratching the Staff of Power off their possessions list, rolling on the Magic Item Sales Table, and converting it to 75K GP, without first asking the DM.
Some NPC now has a Staff of Power? That might influence certain story events!