This one was giving me a headache.
I was trying to enroll Swift Software Group in the Google Apps Reseller program, but the standard application form did not want to accept our domain name “swiftsoftwaregroup.com”. It kept coming back saying “This domain has already been registered with Google Apps.”
I just found a way around this:
Basically, you first have to sign in to your existing Google Apps account and then navigate (in the same browser window) to http://new.googlepartnerconnect.com/Home/enrollment.You should to complete the reseller enrollment from there.