Maybe you got into a conversation with your CISSP instructor at the culmination of your CISSP training course and you casually let it drop that you are using a brain dump to get complementary CISSP study material. Before your IT instructor hits the roof and starts on a litany about how dangerous it is to use brain dumps, head him off and explain that it is legal to user certain brain dumps provided you meet certain conditions.
In its most literal form, a brain dump can be anything coming out of your head meaning, you attempted to write down what you know about a particular topic or field of discipline. Thus, you dumped the contents of your brain about that subject onto hard copy (like a personal diary or notebook), or in a digital document (like a text or word processing document and saved it.) So far, nothing illegal here anyone can write about how he understood the CISSP study material he recently went over. No one can convict you for actually absorbing the information.
Legal profit-making brain dump sites can be found online. What makes them legal is that they create unique content to be sold to other people, so they do own the intellectual property rights to that unique content they developed. Even if the unique content is based on the coverage of past IT (or CISSP) exams or study material, so long as the CISSP study material or CISSP brain dumps these websites sell is of their own creation, that is their own property and they can do with it as they please (even give it way if they are so inclined.)
Why would anyone sell content from the IT certification exams verbatim (in essence, stealing the copyrighted content from the exam creators)? Well, for one thing, it is cheaper. To create your own unique content takes talent, time, energy and resources so some people take shortcuts by ripping off content from other content providers instead. But since you want to start your career on the right footing, buy only unique CISSP study material content so you do not feed the cancer called trade secrets piracy.