Of course it’s worth bearing in mind that anybody that gives their assets out via Marketplace can be victims of Piracy or File-Sharing (posting screenshots of materials etc would be file-sharing really). It’s just that Blueprints and engine-created content is much easier to duplicate.
Honestly though, there’s not much that can be done about it. Blueprints are supposed to be a flexible as possible, and I highly doubt that any blueprint stuff sold on the marketplace wouldn’t need customizing and adapting to suit somebodies project. Otherwise we’ll end up with an awful lot of cloned games out there. Eventually of course the things you have learned and sold will be known by a much larger user-base and sales will decline, but that’s just the life cycle of anything you sell! It has a nice side-effect though, it pushes you to learn more cool things to teach the community 
TL;DR: I wouldn’t expect to be able to make continuous money from a Blueprint or editor-created project. I see no reason why you couldn’t build your content in C++ and distribute it without source code eventually though (when we get that functionality). Of course, Epic may have rules against that too! Time will tell