FYI for anyone that has an Xbox or is about to purchase one online.
I just got an email from my friend at Verizon. They are seeing many people call in regarding the Xbox being unable to access Xbox live.
It has been rumored that Several Hundred Thousand Xbox 360's were banned in November 2009, resulting in their no longer being able to connect to the XBOX Live service. Some of these banned consoles have shown up for sale online attracting buyers due to the deeply discounted price. Problems occur when one of these banned consoles is purchased without disclosing that fact to the buyer.
Xbox owners can get more information on the ban by contacting Microsoft at 1-800-4MY-XBOX (1-800-469-9269).
So if you see a cheap Xbox for sale online. This is likely the reason why.