If you're looking to sell your unwanted goods, there are some obvious choices: Gumtree, eBay, Trading Post. One potential gold mine that's often overlooked is the one that millions of people use every day: Facebook.
You can easily use Facebook to pitch your unwanted stuff to the friends and family members that you're already Facebook friends with, but that limits your audience significantly. For a broader pool of buyers, look for Facebook sale groups in your area.
Search for your state or city and the word "sell" or "buy" to find local groups . If there are none, you can always start one and get the word out in your community. Using a group, you can shop from people in your area, list your own items for sale, and set your own prices. You can even note whether you're willing to ship something, or whether you'd rather the buyer come and pick it up (a useful idea for larger items like furniture).
The big benefit of using Facebook like this is that you set your price and there's no one taking a cut of your sale price or charging you to host your listing. You can set your own preferred method of payment, and you also have the opportunity to become part of a community. Many of the groups are closed, so you'll have to ask permission to join, and others only accept certain types of listings, so make sure you read up on the group you want to join and its rules. Many have strict listing requirements and limits on how many items you can try to sell per day in order to keep out spammers and scam artists.
How to Sell Your Old Stuff on Facebook [Wise Bread]