Why should it be illegal? From a US perspective, I don't think selling words/ideas should be illegal in most cases--especially just because it could be used for doing something illegal. For instance, there are books which describe chemical processes for synthesizing methamphetamine (https://www.amazon.com/Secrets-Methamphetamine-Manufacture-U...). But they are not illegal, because if they were it would infringe upon freedom of speech. Deciding what can and cannot be sold/published is a very authoritarian path to go down. It is better to just outlaw the criminal behavior.