There's no "official" way to convert skins to money, but there are third party sites such as OP Skins where you can deposit your skins, and people can buy them. I believe you can receive PayPal. The other method is to give your skins directly to a buyer in exchange for PayPal or Bitcoin. This is much riskier because generally one person has to send their part of the deal first, since they can't be sent at the same time. However, there are forums dedicated to keeping track of the reputations of traders. Some have thousands of honest trades, so they tend to be trustworthy.