The reason why the light novel " Invader of the Sanping Room " was not sold in bookstores was probably because of the copyright of the novel. According to the relevant laws and regulations, novels, comics, animations, and other works needed to obtain the authorization of the copyright owner before they could be used for commercial operations. Otherwise, it would be an act of copyright violation. As Invaders of the Sanping Room was a relatively well-known light novel, the copyright might have been owned by some companies or individuals, so it was not sold in some bookstores. As for the reason why the novel was finally found online, it could be because the copyright protection of the novel was relatively strict and it was not put on sale on a large scale in major bookstores and e-commerce platforms. It could also be that some illegal publication or pirated books were circulating. As for whether it is out of print in China, I don't know the specific information. However, due to the influence of copyright issues, some novels may be difficult to carry out commercial operation in the mainland of China due to the restrictions of the copyright owner, but this does not necessarily mean that the novel has been out of print.