Microsoft Windows 10 Pro OEM CD-KEY (Global) is different retail?

I found a cheap store selling OEM cd-key, bought and worked.I feel the need to report this deal.
Microsoft Windows 10 Pro OEM CD-KEY (Global) - $15.54 @ SCDKEY. while it is OEM not retail. What is the difference between them?

 

OEM is usually a copy that ships with a machine, while retail is a separate copy you buy from a retailer. They're essentially the same just that OEM ships with a computer when you buy it whereas you'd buy retail directly from an online site or off the shelf.

 

OEM will also take hardware id's from your system and register that product key to that specific system. Any major changes (mobo change primarily) will cause issues with validating the product.