With this recent and huge wave of bans, the second hand market for 360s is going to be fraught with danger. If you pick up a 360 second hand and it has been live banned, or even flagged but not yet banned. then you have no recorse with Microsoft, they don't care if you just bought it, you've lost a significant amount of the functionality, and probably won't have any recourse with the guy who sold it to you either. (other than leave negative feedback, complaing to moderators etc.)
Obviously there are going to be people trying to get rid of their banned 360s, and maybe not being honest about their status, so can I suggest that it be required that anyone selling a 360 should have to include a photo of their username next to the tv showing the xbox logged into say.. the home page on live marketplace.
not perfect I know but it might help a little.
Another option might be a photo of the serial number, so that people could phone microsoft and verify if the console is banned or not, but then I don't know if MS would give that information out.