I looked into my Empyrion keys and it turns out they've actually been used. Just a little update there...
As for your concern on gifting games, most of the games I have are in the form of Steam Keys anyway.
Also an update to my trading issue, someone did end up trading me for Stardew Valley, so I'm all set. I still think it could be a discussion here on the forums though... Trading games on Steam can bring up some issues in trust if you just put yourself out there to strangers. Having a place to do so in a community full of people you trust doesn't seem like a bad way to go about it.