Here it is !
So, after doing some calculations, I think that something like that may work :
1) Magic is a mysterious, dangerous and powerful force : you can have too much magic for your own good...
2) Each character has two more score :
- Magic potential = level + 3
- Magic maximum capacity = level *3
(note : this is for relatively low magic level... You can replace "3" with "4" if you want more magic item for your PC)
3) You can use a magic item power up to your magical potential. Item with higher level behaves like lower level ones of you don't have the required magical potential. IE : a +5 sword behaves like a +2 sword if you are only level 3 (because a magic sword +2 is a 6th level item). If the item don't have lower level equivalent, then you can't use it, or (DM will) you can't control it (IE : the magic carpet takes you where it wants, not where you wants)
4) So, what about thr rich merchant heir, who want to start gaming with a ton of low level magic item ? Simple : if the total levels of all your magic item exceed your magical maximum capacity, then you lose one healing surge per day per exceeding point of magical capacity. When you have no surge left, you start losing hit points, 4 per day per exceeding point of magical capacity. You can't regain those hit point until you have less magic stuff on you than your MMC for a full day.
IE : a level 2 young fighter prince has just spent daddy's treasure on magic item. His MMC is 2*3 = 6. He now has a magic sword +1 (level1), a magic delver armor +1 (level3), a pair of bracers of might striking(level 2). He feel nice, but he senses that magic starts to overwhelm him. If he use more money to buy catstep boots (level3), he will lose 3 healing surge per day. If he tries to wear a ring of invisibility, he will start coughing blood and lighting while the powerful magic is killing him...
5) (optional...) A character knows immediately if an item is too powerful for him to keep safely. Don't kill PC in their sleep because they grabbed a powerful magic item.
I think that such a system allow you to disconnect economic wealth and "stuff" power.