Copyright Infringement of Published Materials

[iamboard]

I run a forum for professionals who have to study, take and pass exams as part of their profession (Like an accountant would have to take the CPA exams). A lot of posts are of the subject matter pertaining to study materials for these exams. Some of the authors of the study materials also post to the forums to help users. A lot of the users purchase study materials from various authors. Many users post to the forum to buy and sell these study materials. Some bad users have resorted to copyright infringement by copying the print materials to pdf files and then re-selling multiple copies.

As a forum admin, how can I stop or limit this behavior so that these authors do not have their materials copyright infringed upon? What methods should I use? If an author who feels he has been damaged contacts me and wants information on the user, am I obliged to provide it to him/her? What can I provide? How do I avoid this whole mess? Even if I delete the offending posts, there still is time for material to be improperly sold.