Can we buy another author’s approved Site Template’s ‘Extended License’ and create Wordpress/eCommerce/CMS theme on it, and sell it here on themeforest?
The Extended License’s terms say
Use, by you or one client, in a single end product which end users can be charged for -
Can someone, especially from Themeforest Team inform if there is any problem with this approach.
What you should do is contact them directly and work out partnership / purchase terms, most HTML designers would likely opt for partnership, usually 50/50% of the CMS versions sales, depending on your agreement.
To reiterate, extended licenses to do not allow you to further develop a template and sell it on themeforest, you’ll need to form a partnership instead.