What sites do use to buy and sell online?

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Do you use online auction sites or 'storefront portal' sites to buy or sell online? I am researching auction and 'storefront portal' sites which allow individuals to sell online with a view to publishing a guide. There are the big ones, of course, like eBay, half.com, Amazon and Craigslist and there are more specialised ones, such as Etsy (Art and crafts), Zazzle (artwork on clothing &c) and Deviantart (photography and artwork). If you know of others, it would be very useful to me if you could post the URL of the home page. Some of the things I would be interested in knowing are: What are your experiences in general of any that you have used, or thought of using, as a seller or as a buyer? Did you find the website user-friendly? Was help for new users good? Is it well categorized and easy to find what you wanted? Did you feel that your items (if you were selling) were well listed? Did you feel that there were plenty of customers for what you were selling? Did you find payment easy and secure? What methods of payment do they accept? Were you happy with the transactions? If you had to deal with returns, was the site helpful or not? Finally, if your comments would be helpful, may I quote them in my review? Thank you, fellow buyers and sellers!