I have an issue with premium WP themes.
There is an inherent conflict of interests. The theme designer wants to sell the theme, not your product or service. The theme developer wants to sell the theme, instead of making sure your site is fast, future-proof and easily extendable.
So as a result, you get lots of flashy themes with all the bells and whistles. They look nice but run slow, break really easily and might not do much in terms of actually selling your product. Also, the code is typically difficult to work with.
There are, of course, exceptions – but you won’t know before you buy the theme.
If you need a small and cheap website set up quickly or if you are having trouble finding a talented designer, premium themes are generally a very cost-effective choice. But you should definitely consult with a professional before picking a theme.
However, if you actually need to sell something, consider investing into a good custom-designed, custom-built site.