“We’re also testing an option that gives video owners the ability to permit downloading of their videos 
from YouTube. Partners could choose to offer their video downloads for free or for a small fee paid through Google Checkout. Partners can set prices and decide which license they want to attach to the downloaded video files. “
As it follows, a new "My Purchases" tab has already been added under "My Videos." Just so you know which clips are already yours.
However, the big questions is: will anyone pay for such a merchandise? First of all, most of the YouTube content is shot in a very poor resolution. It’s not like you’re renting a DVD to watch o a decent-size monitor.
Second, there are already plenty of tools that enable users to download every available clip on YouTube. So, who’s going to pay when he can have the same poor-quality content for free?
Last but not least, there’s one more question that comes to mind: why would anyone desperately want to download content from YouTube? It’s right there, one click away, and )in most cases) it’s not going anywhere!