Green Man Gaming already does it. You have to use their abysmal "Capsule" though, which in my view is worse than not being able to trade a game at all. Any service that allows trading of digital media would use a similarly terrible service, so no thanks.
Not being able to trade things is a non-issue for me - I keep everything I buy.
“The greatness of a nation and its moral progress can be judged by the way its animals are treated.” - Mahatma Gandhi