There are surprising legal precedents on keys resale: https://www.gamesindustry.biz/articles/2013-10-09-digital-rights-are-exhausting-is-it-legal-to-sell-secon

There are surprising legal precedents on keys resale: https://www.gamesindustry.biz/articles/2013-10-09-digital-rights-are-exhausting-is-it-legal-to-sell-second-hand-digital-games https://www.gamesindustry.biz/articles/2013-10-09-digital-ri... It seems that the situation is currently unclear (at least in certain parts of the world), but not obviously illegal.