Where to start with creating the theme for selling? I have the task to create the Drupal theme from the existing Wordpress theme that will be sold on ThemeForest.

This is the first time that I'm doing something like this, up until now I was doing the themes for concrete clients according to PSD files, but I think that I need different approach for this.

Until now I usually did the themes as Omega sub-theme, can I create the Omega 4 sub-theme in this case? Can I use base theme at all or I must create theme from scratch?

I noticed that a lot of themes are based on Bootstrap, is that maybe the best way for creating the theme or you suggest some other way (I know this is maybe the question for opinion but I'm looking for the simplest way for creating the theme and Bootstrap is maybe one of the most complicated ways, so because of that I need your opinion and suggestion).

It would be really helpful if you could direct me to some page or tutorial where I can find more information and choose the way in which I will create this theme.
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.