It's a good idea, considering the popularity of themeforest I'm not surprised to see this. But, if you want to charge 20 bucks you'll need to offer multiple pages of samples, the landing page sample doesn't really tell me what a form might look like with those theme elements, or blog post, image gallery, etc...
That's a good point. A style guide/reference would be very useful to include in the live preview. I'll definitely be adding more to the StackLayer theme and all customers receive updates for free.
Good luck with it though.