Okay, but is it possible to sell steam keys on here? If not, that's fine and I'll just do the alternative :)
From what I personally understand (and you should better check the FAQ to be sure) Steam keys are treated as an extra (or “Goodie”). You can give customers Steam keys, but you have to have a product to sell in the first place. After all: Why would anybody in their right mind go to itch.io just to buy a code to get a game on a different store?
Most developers* who sell their game on Steam and on itch.io simultaneously offer a DRM-free version of the game here and add a Steam key to it.
.* That I have checked out myself. If you want a statistic you’d have to write it yourself.