These keys shoud not be taken as legitimate and legal. Most of these are from people who have access to not for resale keys, intended for development and testing, such as MSDN subscribers. If Microsoft catches him selling keys, the keys will be revoked.
I would always buy from reputable retailers. If they have an actual store, I would ask for the retail box. If you want to save some money, go for the OEM System Builder licence. That is exactly the same as the retail FPP, but comes with a nice certifcate of authenticity which you can put on your case.