I bought a PCAT exam prep study guide from a girl who bought it from Dr. Collins PCAT site, I am trying to resell it now, but he's threatening to sue.

I bought this book off of a girl last summer, I cannot recall any details of the girl as I met her through "kijiji.com". She sold me the PCAT study guide by Collins as well as many other books she bought when she studied for her PCAT exam. I realized I am not taking the PCAT anymore and therefore am selling back the material she sold to me. I am not sure if they made her sign an agreement that she will not resell their book, but Dr. John Collins emailed me that he would prosecute me if I continued to try to sell my study guide because I signed an agreement. After I clearly told him I did not sign any form of agreement so under what basis would he be prosecuting me for, he simply said "cease and desist or face prosecution. Our final words in the matter." Am I doing anything illegal by trying to sell the study guide I bought or is he and his company falsely trying to prosecute me? They will have no evidence I have signed an agreement because I didn't buy from them.