LostSoul
Adventurer
Well magic marts are the direct outgrowth of the Christmas Tree problem. If the adventurers, like all good adventurers, have been successful and filled every single item slot with shiny goodies, in many cases shiny goodies that were good ten levels ago, but useless now, then, unless the adventurers are the only adventurers on the planet, there are a lot of excess goods around.
I don't use magic item marts (4E or so). My setting is post-apoc medieval; shops of any kind are hard to find! This meant that the PCs hung onto their magic items. There might be a use later on; maybe an interesting NPC-turned-henchman could take it. Lots of little magic items.
This recently changed. I now require Wizards to spend Arcane Reagents to level up. Residuum can be substituted, as normal. What's a great source of Residuum?
Magic items.
I don't know what kind of economic impact that would have on the setting, but I think it's in the realm of "now I can handwave away magic item merchants."
Wizard wants to learn a new spell? Gotta go raid a dungeon.
Fighter wants to get a new trick? Gotta go raid a dungeon.
Cleric wants to get a new miracle? Gotta go raid a dungeon.
You don't get to choose what powers you get. You get what you get. You don't gain any unique abilities from leveling up.
Every power you get, of course, is wondrous and weird and a little complicated.
This is highly gamist -- folks who are a fan of storytelling in their D&D probably wouldn't be a fan of it. Similarly, folks who are into gearhead build-making won't enjoy it much.
I think that's a cool idea. It's something I wanted to implement in my 4E (or so) game, but haven't done a good job of. Basically, you gain access to only PHB feats, powers, etc.; if you want other ones, you have to find them in the game world. (As it turns out, this means the DM has to put those things in the game world. That's why it's not working out so well.)
I think you underestimate the power of that rule for non-gamist play. Storytelling guys could have choices that have a lot of thematic value; simmers could have choices that reflect what their characters have done. The systems might have the same base but they'd look different.