Ebay, the famous auctions site. Real auctioning and much traffic.

Half.com, the child of ebay. It belongs to the same company and it is a perfect place for books, movies, games and music.

Craigslist.org, an interesting place for sellers with high traffic and presence all over the world.

Recycler.com, an alternative way for recycling is to get rid of things that you don't want but may be someone also who want them.

Sell.com, a site for selling and buying through negotiating.

Blujay.com, a good site for auctioning.

If you are interested in more, you may open the pdf document "Best Auction Sites" where there are included the top 100 auction web sites.

I found this list in a very recent research and it seems very interesting.