There are some good beginner forums that touch on this topic on NamePros. You can list your domains in the Marketplace section of NamePros to get them in front of domain investors that are here on NamePros, as well as using different external domain selling and auction sites such as Afternic, GoDaddy Auctions, Sedo, Dan.com and others (see the posts at the top of the Domain Beginners section for more info). Hope that helps a bit!