While I believe that @Pippin is absolutely correct regarding how most premium theme developers work, you should know that you can grab any open source starter theme or framework, and - as long as you respect the various component licenses - use it as a base upon which to develop a premium theme.
This can be seen in the comment stream in the handcrafted-wp theme where ZackLive asks
Nice theme, I'm wondering if I can create premium themes based on Handcrafted for selling, thanks
and author Randy Jensen responds:
All the code in the theme is GPL so as long as you respect the license, yes.
*Envato's Official Statement on WordPress Theme Licensing
GPL Compliant since 2009
As the operator of the largest marketplaces for WordPress themes and plugins, Envato is proud to have been GPL compliant since 2009. All themes and plugins sold on Envato Marketplaces have GPL licensed code.