You perfectly can sell GPL code. And you can double-license yourself (provided that you are the copyright holder) as GPL and a privative license. A lot of companies do that, legally and correctly.

You can sell GPL code. Even if you aren’t the author. What you must do is share the code with those customers though.

All of this could be there with the matrix.org protocol. The matrix protocol saves the comments and content in a directed graph, and that graph is copied to every instance, once one views it. It may not scale though. But it has benefits, such as encryption (making communities private or gated when under attack)