You could join the CMCurricula Yahoo group to purchase and sell materials. They are a general CM book group for buying and selling, but I’d be willing to bet you could find a fair number of SCM users there.
You could also start a thread “Looking to buy module 4 books” and then just list what you need. Of course, you’d have to provide a disguised version of you email address for people to contact you since we can’t yet PM forum members, but you would be able to negotiate one-on-one.