Is it possbile to make a website similar to themeforest.net with pre exiting modules? i made a research but couldn't find any modules with similar functionality.
I'd start with Ubercart as the e-commerce module. Its catalog supports imagecache and lightbox2 modules which would be needed for the kind of image functionality on that site. As for the live preview functionality I'd set each one up in it's own sub-directory off of the main drupal directory so each one is it's own drupal site. This appears to be the way that kind of functionality was accomplished for Drupal themes on templatemonster - see http://www.templatemonster.com/drupal-themes.php.
I'm pretty sure you can make anything you want with Drupal. From my humble experience it's very powerfull. But..then again, you have a great advantage if you were a programmer . Good luck !