Is it illegal to buy books and then sell them?

This question involved legal issues such as copyright and intellectual property rights. The specific situation depended on the country or region, the type of books purchased, and the copyright status. Generally speaking, if the book you bought had copyright, then selling or sharing the contents of the book might involve copyright violation. In most countries or regions, copyright protection is limited. If you modify or share the contents of the book after purchasing it, it may not be a copyright violation. However, if the content of the book was sold or shared without authorization, it might involve illegal activities. Therefore, if you buy a book and become interested in the content after reading it, it's best to understand the relevant legal issues first to avoid unnecessary trouble.