Since dnn enforces ssl at the page level all the user controls (checkout pages) within the buynow module will be ssl enabled when you turn on ssl on the page with the buynow module. Since the product detail page is a seperate module you can turn off ssl for the product detail page.

In the buynow module the only page that you wouldnt want ssl enabled is the product listing page all other pages are checkout pages and should be ssl enabled. In order to allow you to turn ssl off on the product listing page we would have to split the product listing page off as a seperate module. The cart already comes with 7 modules and I'm not sure the benefit outwieghs the increase complexity of setup and config by introducing an 8th module to the cart. I think if you're running on a decent server the speed difference is negliable.

All the other modules that come with the cart that can be placed on other non ssl pages; product details module, featured products module, category menu, quick order entry, etc.

See the following forum post for complete instructions on setting up ssl for the cart

http://www.smith-consulting.com/forums/forumid/1/postid/136/scope/posts.aspx#136