Not to be too pedantic, but aren't those typically being purchased under some other license that doesn't apply to you (bulk, oem, cheaper country, etc) and then resold in violation of the licensing terms?

And that's when they're not just "pirated" themselves?

Seems at that point you might as well just skip paying whatever questionable middleman in involved if you're not concerned about abiding by TOS or license.

I haven't read the entire EULA to be 100% sure, but Windows 10 IoT Enterprise LTSC 2021 can be found with MS solution partners so I don't think they would be in violation with M$.