What about in a campaign where there are no magic shops? I myself hate the idea of just being able to go in and buy a magic sword or shield right off the shelf, pre-made to order; capitalism is all well and good in the real world, but this is a fantasy setting.
Even before the dawn of true capitalism, anything that could be bought and sold would be bought and sold.
Its not that Ye Olde Magic Shoppe is a magic item factory, its just a clearing house.
For example, the farmer who is tilling a field that once was the site of an epic battle might find the odd bit of magic here and there...and rather than use it himself, he'd sell it to improve his lot in life with better seed or a new plow. Or a younger, stronger plow horse. Or some fine fabric for a dress for his wife. Or to give his daugter a dowry, or pay his son's apprenticeship fees.
Why do that when he could use the stuff and potentially find even better stuff by adventuring?
1) He may not be allowed to keep arms and armor by law.
2) He may be too old to be a warrior.
3) He may be a pacifist or merely afraid of going into battle. Adventuring
is dangerous, after all.
4) This is his family farm, and has been for 400 years. Why mess with tradition?
5) If he is an indentured servant, tied to the land by debt, selling that bit of magic could be his path to becoming a freeman.
Etc.
Like the PC who sells his Ring of Fire resistance to get a better weapon, the Farmer is selling the weapon in order to improve
his life.
I would have a problem with it. How is that unicorn supposed to handle the money, much less balance his ledgers, without opposable thumbs?
I've seen a puff piece on the news detailing that at least one gas station that has a trained dog that handles the money, taking it from customer to cashier, and back. It doesn't do the counting, though.
And a unicorn at least has the advantage of being equipped with a natural quill or stylus.
They sell the sword, credits in ledgers would be switched from one person to the other, and the actual cash would still sit wherever it has been sitting for years as its value has been transferred from ledger to ledger.
Even in the age of modern, high-tech capitalisim, there are small, isolated towns where promissory notes, checks and similar money substitutes have been circulating for decades. Even though they're technically of zero monetary value- if presented to a bank, they wouldn't be honored- they still operate like cash within that microeconomy.