If you mean selling skins on Steam, it's absolutely true. I do that often to buy some games that are on sale. There is a market implemented in Steam in which people buy and sell their inventory items, however, you can't withdraw the money you make from selling on the market. You can only use it to buy other items or to buy games, and you can't send it to friends either. It's not Steam Credits as @playfulfoodie guessed but normal currency (euro, dollar, ...) depending on the country you live in.