You're looking at a very truncated list of what they said were their source material was. Gygax at the very least was very familiar with Moorcock's work- the alignment system was heavily influenced by it. And again, you can find people buying magic in all kinds of faerie tales and stories from all over the world.
Okay, the statements about breadth of source material, plus variation table to table, makes sense to me. You provide a few examples from literature, of mundane-to-magic commerce, and I'll take your word that you could list more if you spent a while digging up specifics from a wide swath of literature. But I'm still curious how often the protagonist is *spending adventuring loot*.
Juliet gets a Feign Death potion from Friar Lawrence, and that potion is either magic or sufficiently advanced technology... but she's not buying the potion with gold coins which she gained when she fought packs of kobolds, driving them from the outskirts of Verona so that humans can re-settle the area, is she? She's not balancing that purchase against a +1 sword or saving towards a Bag of Holding. She's not *upgrading*, is really my point. She's interested in a purpose other than incrementing her Challenge Rating.
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When I wrote "5E PCs see gold as "useless" in a way which would have *baffled* Conan, Grey Mouser, Frodo, and Elric, as well as Arthur, Robin Hood, Beowulf and Odysseus"...
What protagonist, anywhere in ancient myth, middle-ages-Christian legends, pulp fantasy, Errol Flynn movies, or any other literature before CRPGs, says this:
"I found a few thousand coins in that dragon's hoard, and I might as well toss all the gold and silver into a privy, because no one's selling magic items, and without a magic item shop, there's *no other worthwhile way* to spend those thousands of coins."
I listed four mythical characters from magic-abundant settings, who I would never expect to say that; and four characters from magic-scarce settings, who I would never expect to say that. I don't mean that those eight are the entire list of sources of D&D. Gygax and Arneson borrowed far and wide. (All of my eight are Western; they used Western sources, and they also used non-western material such as djinn and rakshasa.) I'm using those eight as examples that one can go in eight divergent directions of the genre, without finding the premise that "gold, when not spendable on magic items, is worthless".
So, you and others have provided me with some excellent and instructive information and perspectives on magic stores, thank you, but I *remain baffled* by the idea that "gold, when not spendable on magic items, is worthless".
There's plenty of ways to D&D which I understand and happen not to share. To each their own. I get that. Some people love fire-arms in D&D, some find them abhorrent, and I understand both perspectives, without sharing both. Fine - their gaming table isn't my gaming table. However, the idea that "gold, when not spendable on magic items, is worthless", truly baffles me. I can't even. Help, please?