Buying and Selling Registered Domains
In addition to the various methods already mentioned in this thread, I recommend selling or buying registered domains at TheDomainNameAfterMarket.com (a.k.a. TDNAM.com) presented by GoDaddy.com. It is a reasonably priced domain auction site created specifically for this purpose.
When I used to work at GoDaddy.com it was a brand new product growing in popularity for bulk domain owners, sellers, and buyers. I'd be glad to help you but I recently left GoDaddy.com to pursue a more lucrative job in front-end payment processing for acquirers.