Wordpress theme framework?

Hi, looking for some advice/suggestions, and think you might help me out.

We would like to design/dev a set of themes to be used by our clients as well as selling them at online marketplace (like themeforest). We believe having a Wordpress theme framework as a solid ground would be a good idea to build out other themes.

Of course, this can be started from scratch, but wondering if anyone here has experience or knowing a good theme framework to use as a start point?