Negative. In a given segment of a game, in which you make your way from one merchant-dwelling settlement to another, the person who sold all his spare weapons in Settlement A might get 100 gold. The person who sold all his stuff once he got to settlement B might get 110 gold for that same stuff.
I assume you're talking not about an open world? That's a specific case, and in a game where you have no choice of what merchants to frequent the effect of regional pricing would be different (largely diminished).
Since selling random pick-up-ables to merchants is not the only way in which to obtain funds in the game, you're going to have income outside of this, regardless of when you sold those weapons, from all the other stuff you did in and around both settlements, in this example segment of the playthrough. So, let's say that, just doing the other general stuff, you get another 100 gold (off of dead bandits or something, who knows). Well, person #1, upon reaching settlement B, has 200 gold, while person #2 has 210 gold. Person #1 can buy 10 more gold worth of stuff than person #1. Maybe that's a single thing that cost between 200 and 210 gold, or maybe it's an extra stay at the inn, or some potions, etc. You're not even necessarily going to spend anywhere close to 200 gold every time you arrive in a settlement.
PLUS, there are other opportunities to go get gold. If you sold your stuff for 10% less, and you want to buy something that you need 10% more gold for, then you have to first go get some more gold. If you got 10% more, then you don't have to do anything else before buying what you wanted.
Let me rephrase: between two players who play the same way - kill the same monsters, do the same quests - the one who goes for the better deals will have more gold and/ or better equipment.
You're making a similar argument to macman here - players can breeze through, and taking advantage of better deals may just save you time. So it is clearly a balance issue, but one about which you may choose not to make a big fuzz; you're using relativism by saying "if the player could fill his material needs by cutting better deals, they just have to grind less".
You're assuming that players aren't trying to game the mechanics. You're talking i.e. of a story-driven game that people play only for the narrative and as long as they can progress along the main path - either by grinding more or making better deals - they will be content. But for this we'd have to imagine a game with a very strong narrative, and even then it would only apply to a segment of the RPG player crowd.
Right now there is only one case in which I'd sort of agree with you on that point; and that's if the goods/ services you can buy for gold are actually sorely limited. IOW, you can make better deals all you want, and you can grind for gold all you want, but you won't be able to actually put that gold to use. Of course this is objectively speaking a design flaw, and one that would get/ gets quite a lot of RPGers raging.
Not to mention, the buying prices would be different, as well, for various goods. There might even be better/worse deals within the same settlement, as settlements can have more than 1 vendor. Also, prices might fluctuate every in-game week or so, making that whole "player who makes lists of everything in great detail" scenario quite moot.
Wildly adding even more variables to your proposed mechanic doesn't help your case, I think. Rather, the less reliably you can plan your expenses, the less positively challenging the process actually is for the player, and the more dumb luck comes into play. We agree that it's nice for the player to have to make some decisions; "do I buy/ repair this sword for the locally charged price, or do I seek out a better deal?" If you're shaking up the numbers wildly, with fluctuating prices across several merchants in one city with several cities on the map, you're just forcing the player to make a stab in the dark. Stated goal of the mechanic not achieved.
BTW, to repeat myself, it's not as if the trading process in CRPGs traditionally doesn't involve decision making. That's not true. As far as I can see, regional pricing/ fluctuating prices add one more layer of decision making, but at the cost of jeopardizing a lot of other aspects of the game.
And finally... even if one player COULD make elaborate lists and backtrack everywhere and get everything in the universe, how would this be any different from oodles of systems in any other game? Fallout. Hey, a door that requires Master lockpicking! Welp, I'll have to make a note of this, and come back to it later when I get 100 lockpicking. The player who doesn't do that doesn't get what's in there. Does that mean the game sucks for the player who doesn't open that door, and the game's therefore COMPELLING him to both make elaborate notes of all the 100-lockpicking doors AND get his character to 100 lockpicking? Is there no one in the world who ever DIDN'T max out lockpicking and who just went through the game not-caring about those doors? The players that DID open all the doors in the game: did they get 700% more stuff than the players who didn't? Or did they simply end up with more accumulated wealth and extra ammo and such, at the end of the game, than the players who didn't lockpick all the things?
I doubt that in any RPG, the number of locked doors comes close to the number of all purchasable items. This is accepting that you can gauge how much lockpicking you're going to need, which a lot of (most?) RPGs are actually silent about. I accept that RPGs sometimes reward making manual notes, but not to the degree you're implying.
A player who puts more effort into efficiency is ALWAYS going to be better off, at any given point in the game, than a player who doesn't. Should we remove all systems that allow for varying degrees of efficiency?
A game can only be faulted for a system that rewards players if the system itself is accepted as being not central to the kind of game it's supposed to be, or if it's accepted as being not fun by the majority of the target group.
It's accepted that players who are good at building characters may have an easier time than those who aren't. I don't think the same is true about obsessing over store prices.