It literally says so on page 116 of the AD&D 2nd Edition Dungeon Master Guide (original rules from 1989, R&E, 1995).
As player characters earn more money and begin facing greater dangers, some of them will begin wondering where they can buy magical items. Using 20th-century, real-world economics, they will figure there must be stores that buy and sell such goods. Naturally they will want to find and patronize such stores. However, no magical stores exist.
And as I said, the Forgotten Realms setting was already a departure from that orthodox view.
