I'm interested in the answer to this, as someone that has the Plat but no use for this benefit 
When I signed up, in the email link they send you to finish registration, you still need to fill in your (last) nothing, nothing is prepopulated. So I would assume you can sell this. Not to mention, once you have the key card, I don’t see why you can’t give it to someone else. Also, they often allow guests.