Hello there, sorry to arrive like this with what looks like worrying news, especially today. I just found someone going by the nick of "Benedictink" is selling a version of Calilbre for €4.99 on the Microsoft Store.
Seeing as the same person also sells their version of Amazon's free Kindle app (or at least using its characteristic logo) in the same place and for the same price, I suspect skullduggery. Have reported to Microsoft of course.

Merry festivities nonetheless.
It's a ripoff/scam. Also saw for sale "calibre companion", calibre tutorial, calibre documents provider, tutorial for calibre and calibre user manual.

Also see 'moon reader pro' for sale; a scam on 'moon+reader pro' for the android. I got the moon+reader pro as I have installed the WSA (windows subsystem android); acutally the ereader app is my reason for installing WSA & moon+reader does not show up in the windows app version of the Amazon app store.
How is it a scam? Selling software is one of GPL freedoms.
Trademark violation.

Even though the software is available for use, I expect that the Calibre trademark, including the name and logo, is not available to be used as this is the property of @kovidgoyal, or whatever Foundation owns the trademark.

To redistribute, the Calibre software would need to be rebranded and repackaged.

Although, I believe most people would agree that it would be bad form to grab someone elses work and pass it off as your own, without ever contributing to the improvement of the original work.