About the only ones that I know of here in North America that is that cheap is the Dreamspark program, which has other requirements.
most likely those are ripped keys from some program. Did they atleast come with a COA sticker of some sort?
If not 95% chance they are stolen/ripped keys. It may work now, but who knows if they will continue to work or say after 6months it comes back invalid/illegitimate.
There is no way that some random seller is getting that much of a discount, even the major royalty OEMS (Dell,HP,Lenovo,Acer) still pay about $50 per license on average