Not saying anything that hasn't been mentioned already, but chances are good you'd be left disappointed.
May activate, but also may not.
May activate & last an hour/day/week/month/year, but there's no guarantees.
A legit OS from MS costs somewhere in the 80GBP/USD range. They sell to OEM's in bulk for a discount, which is the cheapest a Windows license ever sells for 'legitly'. But I'm fairly sure it's not at a 95% discount.....
You're contemplating buying a license key for substantially less than even MS's favoured partners can acquire it for (who build/ship hundreds of thousands of computers/laptops annually)...........