On eBay and probably other sites you can easily find listings for very inexpensive software license keys for all sorts of programs. Here's a screenshot of an example for Windows 10:
And you can find lots of other titles sold in a similar way.
These prices are too good to be true so I'm skeptical this is legitimate just on that basis. Adding to that: the large number of "new sellers" seems fishy like they are getting shut down & just as quickly creating a new account etc.
On the other hand -- there are sellers that appear to be established, meaning they are at least playing within eBay's rules (presumably). There does not appear to be actual software to download which might mean this is not a scam to get malware on your computer.
So its seems at least a murky situation. Perhaps not all sellers are equally doing something right/wrong.
Are these sales scams or other forms of illegal, or are they legitimate?