So you have some odds and ends, craft items and books you're ready to get rid of, but you don't feel the local charity shop is the right place, or you want to make some money or trade off of them. Where do you do this? Well, depending on what it is, and how cool, popular or valuable the item is, the site you use would vary.
Clothes
Designer clothes are usually best sold on Poshmark or ThreadUp or eBay. If you are interested in a trade situation, Listia is a good alternative because designer duds help you rack up credits and then you can save up for a smartphone or other items that cost more money that your parents are willing to lend you. If you have a bunch of off the rack items worth $20 or more, you can send them to 15each.co (US/Florida) and get credit to buy their items.
Crafts
Say you have some crafty items that you want to sell but don't want to get gouged by Etsy you can try ArtFire or Zibbet , or try one of the many other sites that are specific to graphic art, painting, photography and other niches. See a list of 100+ other sites.
Books
Amazon is the most logical place to start but there is also Abe's Books which can help you find discounted university books, and also get more money that the campus bookstore.
Furniture & Other Heavy Items
How about some furniture or other heavy items that are useful? Craigslist or Freecycle. Listia and ebay also do Local Pickup only so you don't have to ship the item.
Cash Money for Cool Stuff
Valuable items that have world-wide appeal (such as rare Emilie Autumn merch) can be sold through any online marketplace then to get more exposure contact fLeesmarket via Twitter and/or Facebook.