Thanks for the kind words, everyone! The whole team here is thrilled to be able to bring this to you, and seeing so much positivity and words of encouragement is wonderful. 
As long as the Epic content doesn’t comprise most of the content in your Marketplace submission, it’s okay. We want to help people present their work in the best light possible, but we restrict it to presentation\demo purposes for things like Blueprints and code plugins. If it looks like it’s being advertised as basically repackaged Epic content with very little on top of it, we’d step in and let you know, but if you’re doing some crazy mad scientist Blueprint awesomeness that’s hard to explain with placeholder art, drop in some of the IB art to help demonstrate the functionality. The rules are fuzzy and it’s still case-by-case judgment calls for the time being since it’s a relatively new policy, but a good rule of thumb is that if an average person could mistake the presentation materials for what the actual product does, it’s a bit too much.