Sly,
Since you are host I recommend that you get a reseller account at
http://enom.com or
http://tucows.com and then get yourself a program to manage the domains for your self and your clients.
The one I use it called DRAMS you can see it customized here for my site:
http://visox.com/drams
http://www.dramsystem.com
I used to buy domains for my clients through other registrars and quickly realized they were basically resellers as well and I could do the same thing and so can you
If you need help, just ask