I think its just me...I had issues like this with the classifieds plugin on a different site but I don't know why I'm having similiar issues on this site.
The problem is - When I start the registration process and get to the prosite option where you either select "I'm Interested" or "not interested", I select "I'm Interest". I hit the sign up button and it creates a new site anyway, never redirecting me to the pro site checkout page.
I do see this error in the console -
fpdbs.paypal.com : server does not support RFC 5746, see CVE-2009-3555
Not sure if that is related
You can see the problem here http://appgi.gs
I reinstalled the site, disabled all plugins. Redid the sandbox settings.