I want to create a WordPress theme and sell it on ThemeForest. I want to use Responsive theme as the base theme for it? Please guide me. Thanks.
In theory you can, but in practice, probably will be rejected.
You can use frameworks but I don’t think you can use the one you linked as the authors policy states:
“CyberChimps offers licensed Products under the GPL. Some Products are offered free of charge and others, for a fee. Whether modifications have been made or not, you are never permitted to claim intellectual”
“You are not permitted to alter, modify or claim to be your own, any of CyberChimps: copyright notices, trademarks, or other proprietary rights associated with CyberChimps.”
Which means you would need to leave all of the backend and front end copyright etc. in place which you cannot enforce on a buyer of the theme
The other problem with using existing themes is updates etc- what happens when WP updates itself or there is an issue if it is not your own code? You are obligated to support basic upgrades on themes here so you need to know how to do this for yourself