It is possible to sell Scratch projects if they have never been uploaded to the Scratch Website. The creator can then attach any license they wish to the code. Just remember that once you share your project on Scratch, anyone can download, remix, and reuse it as told in the terms of the CC-BY-SA 2.0 license. This means that you might want to un-share your project from Scratch if you are or plan on selling your project. It is probably the most practical to sell either a CD with the project or a compiled EXE/APP. Selling copies of the Scratch program itself is not allowed.
This forum topic contains more information on the subject.