WHMCS: How do I automatically sell & setup domains?

I want to give my clients the ability to purchase a real domain from my multisite and have it automatically setup for them without any direct, manual intervention. I've been looking at WHMCS, and it's integration with Wordpress. It looks promising, but I'm still confused. I've purchased an account with them, but before I go any further and purchase an addon / account from one of the WHMCS resellers, I need to clearly understand how this will work to achieve my goal.

Ideally, a client would purchase an available domain name and have it setup without ever leaving my site. The integration plugin may achieve this, but could it be done automatically without them struggling through the WHMCS control panel?

For mapping existing domain names I'm okay with doing that manually, it is the new ones I'm focused on.

What is your experience with domain selling like, and what would you recommend?

Thanks.