CS2 (CS:GO) skins are some of the most expensive digital commodities, with many rare skins, like the AWP Dragon Lore or Karambit Case Hardened worth a lot of money. In order to facilitate trading among players and make the process easier, Valve introduced Steam community market to their platform. And while the community market makes it easy for players to buy and sell skins to each other, you can't sell CS2 (CS:GO) skins for money on the official Steam platform.
Any money you make goes into your Steam wallet as funds, and while you can cash it out later, it takes a lot of time and can be a difficult process. Instead, if you want to sell Steam items for real cash, you have to use third-party websites, such as Skinomat!