If you are going to make a quality item, then it will sell very well without your own ads.
When my last item was released here, I’ve paid about $800 in its first week for 3rd-party advertisements but only 5% of my purchasers were coming from my own ads and all other were members of ThemeForest.
So it will help you to sell more and I would recommend everyone to pay for it on its own (This way you can at least get some referrals) but overall, nothing can compare with ThemeForest itself
Hope this helps.
If you have Visual Composer with a theme which you’ve purchased from ThemeForest, then you don’t need to activate it.
This option is for users who have purchased this plugin individually. But if you can’t activate it AND you’ve purchased Visual Composer from CodeCanyon, please contact its author here:http://codecanyon.net/item/visual-composer-page-builder-for-wordpress/242431/support
I hope this helps