I don’t see why not. There’s nothing I read in the ThemeForest licensing that prohibits you from doing this. The rule of thumb is 1 theme per 1 client. You already get that.
For showing multiple themes look into installing a demo user using Wordpress in Network mode (formerly known as Wordpress MU). Something like demos.yoursite.com or yoursite.com/demos.
Check out Wordpress Network here … http://codex.wordpress.org/Create_A_Network
Hope that helps. Good luck!