I am terribly confused, I am using Pro-Sites and Domain

I am terribly confused,
I am using Pro-Sites and Domain Mapping with Buddypress and WP multisite.
I want to make it possible for new clients to register their domain name right on the site.

My host provides WHM reseller.

What do I actually need to provide a way for members to purchase their domain name, then a Pro-Site, to which I Domain Map their new domain name?

Does that make sense?