There are marketplace sites where you can list skins for sale. Just make sure to use reputable platforms with buyer protection so you don't get scammed
Selling rare skins requires checking current market prices first so you don't undervalue them. I use https://dmarket.com/cs2-wiki/ to research what similar items sold for recently before listing anything. Rare patterns or low floats can be worth way more than standard versions of the same skin. Third party marketplaces usually take a commission but give you more control over pricing compared to Steam market