Athlonite:

 

Just use a pirated copy of windows 8.1 then freely upgrade it to windows 10 instant legit as it seems MS don't really care as long as you upgrade to 10 you then become a legit windows user.... 

 

a statement based on zero actual facts
MS have made it very clear that ug's from pirated Win will not be considered legit

zespri:

 

Of course it could have been stolen/resold, but it is still original, legit, MS key.

 

No its NOT legit,not as far as MS are concerned. Ive come across this plenty of times.
MS have a history of marking , allready activated Win, as pirated & needing reactivation 3months to years later.
Heck, just go back to XP, installing SP1,2 would be enough to trigger 'pirated,de-activation' on Win installed & activated on pirated/bogus keys.
Same with a customers bogus office he bought on trademe, activated Ok, 3 months later marked as pirated & unusable.

Activation means nothing. Plenty of activated PC's have be de-activated by MS later.
MS arnt stupid, they know when bogus keys are being re-used .

Just because you get away with the install & the activation means nothing, long term. MS can still mark your install as pirated at any time.