Symptoms
When setting redirect for a "buy domain" or any other option in BA> System> Settings> Customer CP, the system uses an address which is set in the store synchronization settings. But this is an internal IP address, which is not accessible from outside.
Cause
The issue is caused by the bug with id #PBA-63584 ("Incorrect redirect URL is generated"). automatically
Resolution
Correct URL manually, set it to the store URL.