Like premitheme said, almost all themes here have demos so you can see how they work live. However, you can’t install them on your own server first, as it’s a digital product without DRM.
As a buyer, unless you see a feature specifically stated or demonstrated in a product, you should generally assume it doesn’t exist. If you have doubts, you can always ask the author a pre-purchase question.
Of course, just about anything is possible if you have the proper expertise; but unless you have those skills, you have to assume that you’ll be able to create the layouts in the demo and no more. You can always hire a freelancer if you want to customize certain aspects of a theme beyond the theme’s included capabilities.