Just in case you hadn't noticed you can no longer buy a Windows 10 licence/activation key direct from Microsoft.
You can still download Windows 10 from Microsoft, but if you need an activation key then you'll have to buy one elsewhere:
https://www.microsoft.com/en-gb/software-download/windows10
If you are buying a Windows 10 key from elsewhere then of course you should be careful.
There have always been scammers selling fake licences, and no doubt they will be more active now that MS have stopped selling Win 10 licences directly.
You might pay for a 'cheap' Windows 10 licence only to find that when you come to register it Microsoft say that it's invalid.
Note that you can (legally) install and run Windows 10 without activating it with a licence at all.
I have a laptop currently running unactivated Windows 10 22H2. (It's even associated with a Microsoft account, it just doesn't have a licence).
There are a few limitations to unactivated Windows - but not many at all.
Firstly you will get an 'Activate Windows' watermark on screen just above the system tray.
On my unactivated laptop it doesn't show everytime I boot, only sometimes.
If you set a light desktop background such as snow you'll hardly notice it anyway. (see below for how to do that).
Can you see it here?
You can't change personalisation such as themes or wallpapers on unactivated Windows.
But you can still right-click on an image file and 'Set as desktop background'.
Other than those minor restrictions unactivated Windows 10 generally runs just the same as an activated Windows 10 does.