Bids4Domains - Allows to buy or sell domains at no extra cost. Free advice available. - http://www.bids4domains.com/
Auction2.net - UK based site that mostly caters buying and selling of domains with .co.uk and .com extensions. - http://auction2.net/
TopComs - Offering a selection of .com domains for auction, along with general domain information. - http://www.topcoms.com/
Afternic.com - Domain name auction site with variety of services like domain news, appraisals, and chat. Recently acquired by Register.com. - http://www.afternic.com/