Hey! Thank you for your feedback, I will edit the article and add your question.
Many games on Steam like Dota 2 and Counter Strike drop these weapon skins via crates that are earned through playing the game. Once you have a crate you have to unbox it using keys that are $2.50USD from Steam itself. You get a chance to get an extraordinary rare worth hundreds of dollars, or a basic skin worth almost nothing.
But games like Player Unknown and H1Z1 you get skins for playing but then you can buy/sell skins through websites like OPSkins.com with real people.