So im guessing your steam items skins which you wanna bet gets converted into coins.
What are 'skins'? Can I buy them with the $5 in my account? Or do I have to buy and play a steam game first, and somehow find the skins in-game?
As what NLNico said, can be traded, bought or received via in game. Some skins cost extremely high. Have a look at
https://opskins.comMaybe I'm a too oldschool gamer and didn't get things right; but, in case I'm understanding it correctly, are there really people around willing ot spend $ 200 to buy an ingame item?
https://opskins.com/?loc=shop_view_item&item=19782566sorry for offtopic but find it a bit shocking

You'd be surprised mate. Seen people spending tens of thousands a month on Clash of Clans (mobile game) and also on DOTA2 items. Havent really played CSgo my self but yeah, there are a lot of people who spend a lot of money on in game items or to speed up in game progression.