If you want to set-up an online store, there are competing opportunities from eBay Stores, Amazon zShops, and Yahoo! Stores.
If you don't want to go the trouble of setting up an online store, everyone knows you can sell items on eBay. It's a pretty simple process and if you have something other people really want, you can get a great price for your stuff. Also, it let's regular folks have the chance to earn a living off the wild-and-wooly internet without opening up a whole storefront (see above). But that being said, it's really just a way to have an online garage sale and I doubt any of my readers actually knows someone regularly making good money from eBay.
I have sold some of our stuff on eBay before and I used to take my books to Half Price Books. But I also got mad when I took my books there, I could take in 30 books and get back $4. I have taken three boxes of business books and been offered $20; it was depressing. And selling books on eBay seemed like a lot of work for just selling a book.
So, after years of seeing the 'Sell Yours Here' button on Amazon.com, I decided to give it a try. Wow! Not only is it super easy to indicate you want to sell something, the shipping is already calculated, the payment method is already handled, the money comes in regularly, and I sell more items quicker than I did on eBay! Honest, this thing really works. Take all of that, add in the fact Amazon.com sells much more than just books, and you've got a winning combination.