According to WHOIS the site appears to be based in the Czech Republic. OEM MS Software can only sold with core system hardware.
And what's more, you will not receive any media, you will be paying for a download. This in itself could not explain the huge cost saving as the cost of the media in proportion to the license is nominal. (When you buy MS software you ane buying a license, paying few the privilege of using it.)
As others have said, it looks dodgy. If you wanted to check it out for legitimacy you could drop an email to Microsoft.