However there is a license recycling market that buy up unused or reassignable licenses from say software shops that have gone under and still have 100 visual studio licenses etc
Microsoft never bothers, and at 3 euro the risk you have that "activate windows" might have a low chance of popping up again is well worth it. Just make sure its not a volume license key and your pretty much good to go.
Is it really pirating when all you get is a key and a legal inactivated copy of windows from MSFT's servers says it's legit?
I bought an OEM pro key for $15-20, and it works great.
Research it a bit more and if you call MSFT support, they go into a loop about how if you typed it in and windows says it's good, it's a legit key from their perspective.
However there is a license recycling market that buy up unused or reassignable licenses from say software shops that have gone under and still have 100 visual studio licenses etc