Ah.. Like a store
That would be fucking awesome. On a CSS server forum I'm on they have that kind of thing. They call it the market
They have cool stuff in there like a glow around your username, a strikethrough on your username, custom username color, steal 75% or 50% of another users' "points", and custom user titles.
They're on vBulletin though, and I don't think it's possible or as easy to do it on SMF.