This is a great idea. Screening clients in addition to developers creates the right kind of marketplace for both sides, one that is focused more on quality work

This is a great idea. Screening clients in addition to developers creates the right kind of marketplace for both sides, one that is focused more on quality work than price. If you screen effectively and still have enough good clients left over, this will bring in the strong developers, which will in turn generate greater interest from clients. Kudos! I hope you can pull it off.