try to pretend that magic items would remain as super rare after centuries or even millennia of adventurers rummaging around and eventually putting them on the market.

But the places they are getting sold into are from time to time invaded and turned into rubble by monsters, eldritch horrors etc for no reason other than to ~~destroy the capitals and allow imperialism to grow again~~ make new ruins for heroes to plunder. See, the invisible claw of magic market regulate itself!