It’s not clear how Apple has been able to remotely disable such devices as the activation process should ideally be required just after restoring the device. It is possible that iOS 7 beta that you have installed on your device has expired, so installing the latest release version of iOS 7.0.2 should fix this issue.
It works for me..
So what did I do?
I started my phone recovery mode.
Connect iPhone to your computer & open itunes (please make sure its latest version of itunes).
HOLD start+home button for 10 seconds after your phone reset itself just release power button and keep holding home button.
iTunes request to you restore your phone which is latest version of iOS 7.0.2.
Press update and relax on your sit.
When your phone restoredsuccessfully its gonna ask you to enter your Apple ID to activate your phone.
If your phone was in beta version of iOS 7 it will work! Otherwise you should contact the previous owner to unlink him/her device.
Thanks..