G2A is a grey market where game keys are resold. I had a lot of extra games in my Steam library since I purchased a lot of 4-packs or bought up multiple copies to give away, if the deal was good enough. G2A supports multiple platforms like Origin, Steam, Desura, or just keys. They charge a fee to list and also when it's sold. While you may feel a bit wary about buying from here, it seems like a good way to unload unwanted games.
To sell a Steam gift, you need to create a gift link. I used one of my emails to receive the link, and then copy/pasted into the listing. Since I just wanted to get rid of the games, I sold for a bit lower than the current lowest price. In some cases, I was selling for a loss. In others, I was actually making a profit. In the end, I think I may have made a little profit.
Once you sell an item, the money goes into your account balance. However, it doesn't become available until 7 days later. Once it becomes available, you can spend it, or sign up for G2A goldmine to withdraw it to something like Paypal or bank transfer. I'm not sure if I want to spend it or withdraw it. I was tempted to get Witcher 3 for 25 USD, but they sold out.