The in game csgo skins economy is hugh. Skins have real dollar value according to their artificial rarity in game.
There are thousands of sites to bet or gamble skins.
Also if you sell on the steam market it costs you 15%-12%. But to transfer p2p costs nothing.
A second thing to note is that the sites profit is in skins and in order to
Liquidate skins on 3rd party market you sell them around 75% give or take, so their reported profits is lower.