I was in the collectibles business for a bit last year, and I found that there were all kinds of places to sell your wares. Depending on the item, however, some sites were better than others. Here are the sites I have found to work best for each category of collectible that I sold:
eBay - Due to high fees and shipping costs, this may be the best avenue for rare books and items that ship easily. For more information on opening an eBay store (which can save you in overall fees by paying for a monthly membership), see our "Should I Open an Ebay Store?" article. We have, however, done well to sell china and rare tin pieces via the site.
Etsy - If you can "stage" a collectible to look pretty as a picture, then this site is for you! Many of the 2nd-hand items on Etsy are photographed to look like pages in a magazine, which is why the sellers can charge so much! (They are selling a feeling and a look, not just an item.) Fees are more modest than eBay, but competition is getting fierce on this site. Your best bet is to sell something on Etsy that hasn't been sold there before.
Amazon - This site specializes in new items, but if you have a 2nd hand book, you can sell it on the site. Textbooks, educational materials, and first-editions can go for lots of money here, but there aren't as many people looking. You may find your book sitting there a long time before it finally sells.
Craigslist - This is one of the best ways to get rid of your item locally, as shoppers scour the listings daily. Be sure to include photos in your listing -- which is sure to make it stand out -- and provide a solid price that no one will guess about. (As always, we recommend being very cautious about meeting anyone you find on the site, as well.)
Facebook - Now one of the newest (and, in my opinion, best) ways to sell your used goods, Facebook can help connect you with people in your local area that you can possibly even know before you trade. Check out my recent article on Wise Bread for all the details on how to sell on Facebook!
Your Blog - Some artisans have done a very good job of housing all of their listings (and even a shopping cart function) on their Blogger or Wordpress blogs. Even if you can't set it up to accept real-time payment transactions, it's possible to accept email order and then issue invoices to be paid via PayPal. Your blog should be updated as soon as an item has sold, to avoid confusion and prevent any mistakes with your customers!
What sites do you use to sell your used items? We'd love to hear about what works for you in the comments!