I would think you would have to contact Comcast and have them troubleshoot your cable box. Not sure how they'd react to you having purchased one of their boxes off of EBay.
When my Comcast cable modem died, I plugged in one that I picked up at Radio Shack for $10. Everything seemed normal, but I couldn't connect to the net. Called Comcast and let them know. They had to configure something on their end to get it to work (something like registering the unknown modem). I wouldn't be surprised if you have to do the same thing since it's an out of area cable box.