Is The Windows 10 Key and Office 2016 Serial Number Sale on the Net Genuine?

Is Windows 10 OEM Key under $30 Legal? Recently I intend to install Windows 10 on a new computer, because the software to be used should be bound to the computer’s motherboard (should be the Windows 10 Key). So I bought a ASUS UX305UA Laptop the day before yesterday, but I did not see Windows 10 key on the package box, so I e-mail to…