Fair enough.. but I still see the 'retired adventures, guilds, curio shops, etc' combined as being similar to a 'magic shop' that shortens or makes gameplay easier. No need to make the characters act out going to each individual seller when they could just buy from the one easy source... or maybe see it as these NPCs in game having a place they congregate to sell their wares (not really suggesting the explanation fits every scenario, but .. yeh, I think you get the idea).
At the end of the day, I don't really see much difference in the reasonings/methods behind allowing players to buy their magic items; whether straight shop, specific NPCs, searching, making, hiring a creator, etc. At the end of the day, they're still performing the same fundamental action; passing over goods for gold.
How the DMs incorporate this into the game varies a lot. And I think many of us have fallen into a ditch trying to argue for one method over another for getting the same result. Like deciding whether you should pitch, throw, toss, hurl, or lob a ball somewhere; it does the same job regardless of the explanation.
Having seen a new post, I'll make this addition:
I assume the guidelines were set & understood and this isn't a brand new game that the OP is running with this complaining player.
Players aren't always reasonable. When things don't go their way, whether via bad rolling, an ambiguous wording of the text (and incorrectly chosen feats), different opinions of game styles, etc.. players can be pretty irate (but mostly are great to play with).
Just simple things can cause big arguments; such as one player in my group thinking that demons are not (cannot be) inherently evil; there can be good demons (don't argue against this, I'm not trying to say they are good). Much the same as the same player insisting that all items should be available in the shops at all times; and that I should pass out more gold for the encounters (when I am doing it by the book; random treasure/gold generation).
It comes down to different expectations. One player might like to buy from a shop, while another might prefer a story based approach with rolling to find and acquire specific items. Unless everyone in the group has the same expectations (or no expectations; being new to the game), then there are going to be some disagreements. But hopefully nothing that cannot be sorted out. God forbid you have to kick someone out of the game.