WordPress is licensed under GPL, which according to WordPress means that all WP plugins must also be licensed to be GPL compatible. There is debate about that, but almost all of the plugins in that person’s GitHub account are explicitly licensed under GPL. Under GPL it is technically legal to redistribute (including selling) the actual … Read more
Warning: This is not legal advice. The only definitive answer you can get on a legal subject is from a lawyer trained in that area. If your defence in court is that you read a stack exchange answer that will not hold up. Having said that I’ll try to answer using information from the FSF … Read more
In addition to the other two suggestions, there is another possible approach: move all of your custom-app functionality out of the Theme, and into a hosted web service, to which the Theme connects via API key. That way, redistribution of the Theme itself does not impact your custom app-based business model, because the app would … Read more