Question: TF2 keys for Humble game gifts? How?

That's a new one on me. Sounds like it could be a system that a trader worked out for selling game keys through an automated exchange like TF2 dispenser or similar, but that kind of thing isn't necessary.

I used to buy a lot of games for keys on steamtrades before there were so many restrictions. I never had any problems but be wary of scammers if you are trading for stuff outside of the trade window like CD keys - steam support won't help at all if you get into trouble. Only trade with people you know really well or people with very high rep, and then check at least a few of the people who left rep to make sure they look legit.

The "Empty Crate" you are referring to is a generic TF2 item box. They drop constantly, and unless you want to blow a key on them, they are mostly worthless (unless a rare box). Some people like to give one of those TF2 Boxes in exchange for whatever they are trading for so it doesn't look like a one sided trade. It was believed that if an account receives or sends too many one sided trades that Valve will think something fishy is going on and ban the account. In reality, I have never heard of this ever happening, and I have done plenty of one sided trades. I used to do the crate thing, but now I don't bother.