If the book has been used, the copyright page and the copyright notice should be marked as used so that the copyright owner can detect the violation. Reselling a used book in such a situation could be regarded as copyright violation. According to copyright law, any act of copying, distributing, displaying, performing, broadcasting, translating, or adapting a copyrited work without authorization is a copyright violation. Therefore, if you sell a book online that has been marked as used by the copyright owner, it is an act of copyright violation. In order to avoid any copyright violation, it is recommended to contact the copyright owner and obtain their permission before selling any books.