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.

The iconic StatTrak!" M4A4 | Howl is possibly the most expensive CS:GO skin and certainly the rarest. It's a contraband skin (the only one of this rarity), which means that you can't get it by opening cases anymore. In fact, the only way to obtain the Howl is through trading it with other players on skin marketplaces.