Quite recently Microsoft started banning XBox consoles from accessing XBox live for modifying the console to play pirated games. Turns out that many gamers are now getting rid of their banned XBox consoles for as low as $40 on Craigslist.
The XBox ban is currently only for the consoles and not user accounts, so the users selling them can buy a new one and start using it with their old account.
If you are planning to buy the cheap banned consoles on eBay or Craigslist, please be aware that you will NOT be able to access XBox Live at all, so proceed with caution.