Where to start with creating the theme for selling?
Themeforest audience is mostly end users. So, it does not matter what base theme you use as long as it makes you more productive.

That said, unlike Wordpress where you just upload the theme files zip, you will have to create your own distribution with the latest stable Drupal core, the set of contrib modules you want to theme, the base theme(if you are using one) along with your theme files and assets.

I suggest you start with a base theme of your choice rather than roll out from scratch. Its a huge time saver and most base themes come with sensible defaults, especially Omega 4.