Is there any module which can be used with Drupal 6.x for adding E-commerce especially to sell things like video, audio, e-books.
There is one old module for Drupal 5.x called Quickfile. Is there anything new like this?
Or what are modules required to achieve above stuffs.
Definitely Ubercart. There are dozens of payment plugins available. It includes the ability to sell any kind of files securely by associating them with products that the customer gets access to upon a successful checkout.
There are also some add-ons for doing things like selling license keys, if that is relevant for your needs. Modules for it can be found on both Ubercart.org and drupal.org.