Joe has Windows 10 and it says he needs a key. He's seen one at "MercoSoft" that's cheaper. Why is that? Leo says he suspects that "MercoSoft" isn't really legit and might be selling the same key over and over again. Or they're an OEM key. And you can bet it won't work. It's also technically a violation of Windows terms of service. So avoid that.
You can buy your Windows key directly from Microsoft from within Windows 10. It should cost about $200. But you can buy an OEM Key from NewEgg for about $140-160 and it'll come with a physical certificate. NewEgg sells them for people building their own PCs. That's OK.