As I mentioned previously, it is legal to resell keys from scrapped PCs in EU and UK. However, most of these online keys are sold by Chinese scrap dealers who receive the PCs from EU, UK and Australia for recycling. If I remember right, they used to share a photo of a scrapped PC with the sticker (not sure if they still do that). That is the reason you will get licenses for fixed office versions and not for Office 365.
They do it mostly as UK sellers for legal purposes, so I think they are somewhat concerned about following the law. It is anyone's guess though if they sell a key only once because if they sell it multiple times, it is bound to get blacklisted.
Companies can legitimately sell used software licences for downloadable software following a landmark decision from the European Court of Justice. The case means customers of software companies that no longer want to use the product could put the licence up
www.alphr.com