From the perspective of the commerce that eCart will handle for you the site would be very similar to any physical goods site. You would have product listing pages and a way for the user to get more details about the video. On this page the user would be able to add the item to the cart and then proceed to checkout. Once the transaction is complete you will then be able to deliver the video.
How far along in the process are you and what specifically are you looking for guidance on?