Is he doing it right?

seems fair to me

Good deal, I'd do it if I had it.

Yeah

Seems like a perfectly fair deal.

I would be wary of accepting it though, even as a joke. He might be trying to trade a (possibly hacked/leaked/scammed) key for a legit steam gift in order to keep his acount from getting banned, or a Russian language copy for an international copy, or something like this.

Not accusing anyone of anything, but no one has anything to gain from such a trade, so why even risk it?

Maybe it's only a typo. He was thinking about Space Marine, so he wrote - Space Marine.

No. He's ripping the other guy off. Report him.

seems legit

Most definitely. He'll get a great deal out of it.