Of course, the first place to go for buying and selling used curricula and books is CathSwap.
Other resources for used books and texts:
Home Schooling Unlimited's Free Classifieds
Homeschool Used Curriculum Sites
Addendum: Mary Margaret reports that amazon.com's policy is similar to eBays: "Solutions manuals. Manuals or teacher's editions that provide answer keys to student textbook editions are prohibited."
I think that our best bet is to stay with the homeschool exchanges.