It's not illegal to violate terms of service, at worst it's breach of contract. What policy does it even violate? As best as I can tell Windows licenses are transferrable by first owner provided they aren't installed on another machine.
Copyright infringement? The only thing being purchased is the product key, from an online reseller. The software is downloaded from the Microsoft website. It's not an infringement of copyright to purchase a software key from a company other than Microsoft, it's not even a violation of their terms of service as best as I can tell.