In my experience, games have actually roleplayed the shopping experience... once, maybe twice. To see how the characters go about it and so on. But as you say, it just takes vastly too much time, and more importantly, it isn’t fun after the first or second time. Because it becomes a chore: it sounds just like how I have to make up lists of things I need and shop around for the best price and make sure the people I go with are reputable (and that their price isn’t indicative of a lousy product or service). That isn’t fun in real life, but it’s important. It’s definitely not fun in a game, and it certainly isn’t important either. So I’m right there with you.
That said, others can have different opinions on what is fun. I haven’t run into anyone who really feels like this is what they want to focus a game of Dungeons & Dragons on, but they could be out there. But I would caution, then, that even if this is what you want, you still have a mismatch problem: Dungeons & Dragons doesn’t support this very well. I mean, usually you go, you roll Gather Information to find what you’re looking for and/or what’s out there. Then comes Appraise (itself an annoying tax that adds precious little to the game), and hope you roll high enough to know what the thing should be worth. Finally, you roll Diplomacy to try to convince the shopkeeper to actually sell it to you for something like that price. Again, hoping you roll high enough. Coming up with in-character statements to portray the roll. Having the DM make more-or-less ad hoc judgment calls on whether or not the roll plus whatever you said was good enough. Rinse and repeat for every item.
None of this really gets anywhere interesting. I mean, when you look at things that Dungeons & Dragons really focuses on—mostly combat, some on exploration—the scenario is different every time. Enemies have different attacks, different resistances and weaknesses, the terrain comes into play, and so on. Traps aren’t as fleshed out—a lot of it again just comes down to hoping your Search and Disable Device and Open Lock checks are good enough—but there’s still some going on.
But for haggling, there’s just Appraise, and Diplomacy. That’s all you get—because Dungeons & Dragons is not a game that focuses on this. It claims to be the be-all, end-all system for all games, but it really isn’t. It’s for dungeon-delving and dragon-slaying and it really isn’t designed for much beyond that. If your game isn’t focusing on those things, you are probably using the wrong system.
And if you are going to have to haggle for every single item, you are going to be playing a game that focuses on that. There won’t be time for anything else! That will be almost all of the game. Because D&D 3.5e characters demand an enormous number of items. There’s a ton of mundane gear they’ll need even from 1st level, and then as they level there are just tons of magic trinkets and gear they need. I mean, this is 3.5e’s depiction of the consummate adventurer:
(Dragon magazine vol. 342, Ecology of an Adventurer)
Just look at all the packs and gear he’s got strapped to him! This is what an adventurer looks like by mid-to-low levels.
Which brings me back around to your real question: how should this get handled?
The idealized magic mart with exactly what you’re looking for is the obvious answer, and in my experience most games will include something like one sooner or later just because it saves time and it wasn’t worth expending more effort than telling the players “deduct the appropriate number of gold pieces from your sheet and write in the item; now let’s get on with our next adventure.” Sure, it’s lazy, and sure, maybe it’s “easy,” but again, this isn’t where the focus of the game should be, in my opinion. This isn’t where the challenge is supposed to be found. Leave special effort in finding items for plot macguffins or truly special loot.
But if you wanted to avoid that, there are other ways to handle it. The game’s recommended system for this is “Wealth by Level.” In short, the game says that characters are expected to have accumulated a certain amount of wealth by the time they reach each level. “Wealth” here is defined by the sum total value of the stuff they own—which is not how much you paid for it. The game doesn’t suggest tracking income or expenditure per se—just the end result. So if you were at your wealth by level, and then overpay for some item, the game would now say you are now under your wealth by level. You gave up more gold pieces than the thing you bought was worth, so the total value of your gear and gold is less than it should be.
And what does the game say should happen then? The DM is supposed to fix it. You are supposed to collect more loot than you otherwise would in the future. Not immediately—overpaying can, and should, lead to a temporary shortfall of funds. But over the course of, say, the next level, it’s supposed to get fixed. And the DM does this behind the scenes by adjusting rewards and loot accordingly. It doesn’t have to be magic marts. It could just be loot piles. If you overpay on your belt of giant strength and now cannot afford your cloak of resistance, the DM can—should—include a cloak of resistance in the loot of the next dungeon or so.
In my experience, one of the best ways to handle that is wish lists. The DM knows what the players want—what the players consider to be worth their printed value—and so has options for adjusting loot to get them the items they need. It allows the game to work smoothly, and it eliminates the “metagaming” of magic marts.
And, as a final word, let me emphasize that I did not misspeak in the previous paragraph: we are talking about the characters’ needs here. The entire game is designed around having magic items, having a lot of them and having the specific ones you need. It does not function correctly if that doesn’t happen.
If all of this sounds like not what your DM wants from the game, again, he should very seriously consider running a different game. Many, many games handle this sort of thing better and allow the DM to get what they want from things without causing all these problems.