Trading plays an important role in the CS2 community. Many players enjoy the collectible aspect of the game, and every player wants to fill out their loadout with rare CS2 skins. The Steam platform also features an integrated trading feature, known as the Steam community market, which allows users to trade items from many different games with each other, CS2 included.
And while trading on Steam community market is very comfortable, you won't be able to earn real cash by selling CS2 and CSGO skins there. If you want to receive real money for your items, you'll need to use third-party trading sites, like Skinomat.