 So, you're trying to get your stuff on the Unreal Marketplace, but when you move the default stuff to the demo folder, every time you start your project, it's a blank screen. Not a problem. Any time your game does not start on the environment you want, you go up under Edit, Project Settings, and under Maps and Modes, set your default maps. You're done. Now when you start your game, it should be exactly what you set it to. Hope that helps, and as always, I hope you have a fantastic day, and I'll see you around.