A regular country noble wouldn't have much in the way of what we call "cash", no. But owning a handful of villages and a dozen hamlets, i.e. the output of hundreds of people, will add up. I imagine such a noble could easily fund or borrow 1,000 gp.
Being a powerful ruler in a great metropolis of tens of thousands of inhabitants would amount to a "net worth" of perhaps 100,000 gp.
Think of a high level spellcaster with the backing of a country-spanning church or guild and we're talking 1,000,000 gp, easy.
Too many fantasy games simply ignore economics of scale. While it is understandable that game designers are only fantasy gamers with little to no understanding of economics (or they'd be in a different business than rpgs!
) it is still inexcusable to present a church catering for thousands and expect players to find "we have a grand total of two (2) healing potions" as anything resembling the believable.
I realize it is boring and "too much like work" to add up the number of pigs, smithies and properties that make up any settlement. But if you think about it for even a few minutes, you'd quickly realize that the sums (if only in the books assuming a non-cash civilization) involved are pretty huge (compared to what a low-level adventurer carries in his pocket).
It is the same with the reality-defying claim that even low-level magic items should be "purchasable only by the wealthiest of nobles".
Sheesh, if anything, if the noble can't buy the item for money, he can buy
people for money.
Meaning that if I am a rich merchant desiring your magical sword, how much do you think it would cost me to hire twice the mercenaries of double your level to kill you and take your stuff?
Far less than what my offer to purchase your item and let you live in peace, that's why.
TL;DR: If the game doesn't want PCs to buy magic stuff from Walmart, that's fine. But don't pretend as if there wouldn't be some kind of magic item economy in a highly magical society like those of Forgotten Realms, say.
Not providing any guidelines only means not helping DMs with what they then must do themselves, with all the risks of unbalancing and errors that entails.