You can sell CS:GO skins in third-party skins marketplaces. You should choose a trusted and well-established site for that to be sure that everything will go smoothly.
Choose a trusted CS:GO marketplace, sign in with your steam account, enter your account details (trade URL, email, etc.). Now you can start selling CS:GO skins. Select skins that you want to sell and select the price for them. You should list them for lower price than on the steam community market. Check prices of similar items on the website and compare them with prices on the steam community market to have an idea of how much money you can get for your skins. After you selected the price for your skins, you can list them. From now you'll have to wait for a buyer. After somebody buys your skin, you'll receive the balance, which you'll be able to withdraw for real money. Before you sell your skins, be aware that some sites might require you to complete the KYC (Know Your Customer) process before withdrawing funds. The most common withdrawal options are PayPal, Visa, Mastercard, Skrill, Payoneer, Webmoney, Bank transfer, Cryptocurrencies (bitcoin, ethereum, litecoin and more). There may be some fees when selling, most likely it will be up to 10%. So make sure to check the selling fee before you list your items.
Buying CS:GO skins is very similar, you have to find a trusted CS:GO marketplace, login, and fill in your account details like steam trade URL. After that, you can deposit money and search for your favorite skin using the search bar. Purchasing skins is one-click and you'll receive a trade offer instantly if the skin you purchased isn't trade banned. If it is, you'll have to wait few days, before you can withdraw it.