A great many XBOX 360's were recently "banned", in that their crypto keys have been revoked. This means that the console itself is banned from XBOX Live's service. Many of these 360's, supposedly all of which are modded being the reason for their revokation, are winding up on the used market and you can count on some folks trying to dupe their buyers, either by omission or blatant deception, into purchasing an item they would otherwise not have purchased should they have known or understood that the console in question cannot connect to XBL.
DailyTech - Xbox Live Ban Reaches 1 Million, Microsoft Tells Them to Buy a New Console