Steam take 30% of the purchases made via the Steam Store. If you sell a game on Steam, you can redeem as many Steam Keys for your game as you wish. Those keys are sold at 100% profit to you, Steam dont take any.

Interesting. So you can freely sell keys outside of Steam and they don’t mind?

You have to sell it for the same price, but yes. Most games on the Humble Store give you Steam keys, where Humble takes 25% and Steam gets nothing.