I know that Moniker does this exact kind of auctions. You could just start at http://domainauctions.moniker.com/how_to_sell and try to find them through their website. Or email my user name at gmail.com and I can ask internally (they are owned by the same company that owns my employer) for who you can get in contact with there behind the scenes. Of course I may be told that people are supposed to go through the website, though that is less likely if the domain name is a really good one. :-)

Thank you!