Steam lets you sell Steam keys directly or via a third party and they take a 0% cut. That's how places like greenman gaming, humble bundle, etc work. You still get to use all Steam's infrastructure but pay no fee.

that's a mechanism intended to grant keys to reviewers and such, you can't use that as a primary transaction mechanism. steam has an unspecified threshold at which they will kick you off the platform for abusing that, they aren't there to be a free delivery platform for external sales and if it makes up a plurality of your sales they'll pull the plug.

you'll note humble bundle isn't literally the only sales those publishers make on those games, or even a majority of their sales, and it's also something that's specially blessed by valve. if you pull that as a random shit publisher not through humble bundle you'll get kicked off.

But you can sell steam keys with Valve taking a 0% cut.

See the 2 sibling comments 10 mins before you