Again there is no one single correct answer, it will depend on many variables. but if you are selling individual video clips, you have to have one of these.

What do you use? What do you like about it? What do you dislike about it?

I've already bitched ad infinitum about osCommerce elsewhere (mostly in other forums), so I will give it a rest for now. It might actually be best for people who are not afraid of PHP programming. People who are not comfortable with programming challenges should run far, far away. But it is free!

The best shopping cart I have ever seen yet is the one on C4S. You never leave the video page, you never have to sign up for anything (that is a real death blow to clip orders) until checkout, it keeps a running total--damned near perfect as far as I am concerned.