Is it illegal to resell used test prep materials (i.e. books) I bought from students who took the courses?
1 Answer from Attorneys
Generally, no, it wouldn't be illegal.
Once a printed book is sold, the scope of enforcement of the copyright relating to that work is legally said to be "exhausted," and the tangible book is free to be sold to others without restriction.
Beware, however, that it has become customary for owners of copyrighted works to refrain from selling electronic embodiments of the works (e.g. e-books) outright, but rather to grant permission under license for the works to be displayed only on the user's computer(s). Some such e-books are locked down by a digital-rights-management (DRM) system, and it could violate the copyright owner's rights under the Digital Millennium Copyright Act to "jailbreak" such DRM schemes.
Moreover, certain academic tests have continuing value only when the questions are kept secret, so that the tests can be re-used for another crop of students. There is a general protection for that sort of confidentiality. In the unlikely event that the "test prep materials" to which you refer contain such protected content, also beware.