Not really true.
3.x the rules for pricing and making Wands, Scrolls, and Potions were in the PHB. You could make and price those items without ever glancing at the DMG.
Meanwhile the DMG had rules for population centers and for what you could buy in population centers.
“Every community has a gold piece limit based on its size and population. The gold piece limit is an indicator of the most expensive item available in that community. Nothing that costs more than a community’s gp limit is available for purchase in that community. Anything having a price under that limit is mostly likely available, whether it be mundane or magical. While exceptions are certainly possible (a boomtown near a newly discovered mine, a farming community impoverished after a prolonged drought), these exceptions are temporary; all communities will conform to the norm over time.”
So, magic mart is IN THE RULES. Magic mart is assumed in 3.x because the rules TELL YOU that there's a magic mart in any large city. This wasn't an accident, the rules didn't change from 3.0 to 3.5, the designers EXPECTED play to work this way.
Similarly, when D&D next has playtest rules that TELL YOU that an easy encounter has 0.755 magic items on average and that an average encounter has 1.31 magic items on average and that a hard encounter has 2.28 magic items on average then I don't believe them that magic items are rare. Rules trump fluff. After a few encounters I expect Christmas trees because I read the rules and that's what they tell me.