These cheap keys are usually from MSDN subscribers.
MSDN subscribers can generate an unlimited number of keys at no cost, but they are for use by the subscriber only. Selling these keys is an abuse of the program. There is no resale here and you don't really get a license, you get a key.
The "respectable" resellers do keep paperwork where given license comes from. Just because it is legal to sell them, doesn't mean you cannot be audited what exactly are you selling (once you attract the attention of powerful people).
So while some might come from MSDN subscribers, they often come from unused enterprise ones. The MSDN sellers are creating a big risk to themselves.
