When people argue about the best way to legally purchase a standalone Windows licence...
As far as I'm concerned, if it activates, it activates. If it was a big deal they wouldn't have offered W10 free for a year. There are loads and loads and loads of keys floating about.
If you really want to do it completely by the book, then yes, @ITguy2016 is absolutely right. But in all honesty, nobody really gives a hoot; legally or morally. Not even Microsoft. They make their money back tenfold on the OS from data mining and advertisements anyway.
The OP asked:
"I was wondering if anyone knew weather these sorts of sales are legitimate or not?"
The answer to his question is, IMO, no. Just because a key activates does not make it legitimate.