Selling domain name with Pro Site, Domain Mapping & eNom

I have a couple questions regarding selling domain names through Pro Site, Domain Mapping & eNom.

1. When a user is registering a new site, at what point do they register their domain itself? Does it happen during the sign-up process or after they have registered their site?

2. I have a site that's been live on my network before Pro Site was enabled, its domain name is registered at GoDaddy. How can I move that site and domain into the Pro Site, Domain Mapping & eNom billing system?