Okay, 2 cents worth - why do Wizards, Artificers, etc make items. By the time they can, their sitting on real power and real adventurers are paranoid psychotics that don't like to share. So their adventures, and some may want to help their gimped buddies who saved their bacons way back when they were level one. Doesn't explain why they make so many. Certainly not enough to run magic marts in every city.
And using the argument that any well prepped caster type knows about Sigil, all buyable equipment, etc, they also know how to make said money and when it becomes feasible. Some may be made as rewards, payment, etc, but again Magicmart tm isn't going to spring up. Those items usually get handed down or in some cases, pried out of the hands of the dead.
Magicmarts are a game balancing mechanism, keeps non-spellcasters theoretically on par spellcasting peers. But realistically, I doubt you can justify it economically or psychologically.