This is a cool idea. I've been experimenting with a similar model for design. Undesigned[1] will redesign or prototype a single page in one week for $1500.
In your case, the greatest struggle is going to be supplying the marketplace. It makes sense to offer the service at unsustainable margins for now while you seed the market, so I wouldn't worry too much about all the comments talking about how unsustainable it is, and instead focus on the ones talking about the difficulty of building a marketplace.
You're on good tracks, this is one of the things 37signals did in the beginning.
"
Want to make one of your site's pages better?
Check out 37express: 1-page redesigns in 1-week. 37express is perfect for companies that need one more design comp, rapid professional prototyping of a wireframe or early-design concept, or just a quick, hassle-free, "how could this page be better?" creative spark.
"
In your case, the greatest struggle is going to be supplying the marketplace. It makes sense to offer the service at unsustainable margins for now while you seed the market, so I wouldn't worry too much about all the comments talking about how unsustainable it is, and instead focus on the ones talking about the difficulty of building a marketplace.
1: http://www.undesigned.io