Windows 7/8 keys for 10-20 $ . Are they genuine and should 1 buy them ?

Hello ,

Please check the link

[H] Windows 7 / 8/ 8.1 | Servers | Office | Visio | Project | Visual Studio [W] PayPal,Credit Cards,AGC,BTC,Google Wallet,Venmo,Square Cash : microsoftsoftwareswap


People have been reporting they work great.

But the question is, are they legal and should 1 buy them ? What are the chances that MS might block them in future ? Otherwise please tell, what is the cheapest price where 1 can buy genuine windows legally through any offer or deal ?

Thanks in advance