You can always sell CDs without breaking copyright, you can't sell anything you acquired from GOG according to the TOS. A GOG licence is always personalised. A

You can always sell CDs without breaking copyright, you can't sell anything you acquired from GOG according to the TOS. A GOG licence is always personalised. A Book or Record is not. "3.3 Your GOG account and GOG content are personal to you and cannot be shared with, sold, gifted or transferred to anyone else. Your access to and use of them is subject to GOG’s Privacy Policy and Code of Conduct which are updated or amended when necessary." So far the TOS of GOG and Steam forbid resale of licensed items, if that's leagal or enforcable is another story. https://en.wikipedia.org/wiki/First-sale_doctrine https://en.wikipedia.org/wiki/First-sale_doctrine