Hello
Can we create campaign in Google Adwords and sell ads for PSD and HTML templates: Do we’ve permission to do that? 
I highly doubt it unless an author is intentionally asking and employing you to do it, given that they are not your files to promote.
If author wants to sell ads for themeforest themes, then? Is he/she permitted to do it?
AMK
It depends how they go about it but various authors do use paid promotion to market their products.
You can use the affiliate program to generate traffic etc. to Themeforest but not to specific products without the authors permission. (imagine if they were running their own promotion and you created something to the same product - think of the confusion or worse that this could cause).