If you wish to sell CSGO skins you can do it at Steam Community Market, third-party marketplace of your choice, and at Skinwallet. Steam Community Market's biggest advantage is that it is practically impossible to get scammed. The prices are also quite high.
MONEY now has a new section: Market. It works similarly to the Steam Marketplace, where you can sell your skins and buy new ones. The difference is that money you make by selling skins on the CS. MONEY Market can be withdrawn.