I think the godaddy premium domain list is picked from buydomains and sedo. Maybe those domains have been officially appraised.
Actually anyone with a godaddy account can list their names for sale as a premium but you have to pay a fee if im not mistaken.
Godaddy is a member of Fabulous.com's DDN (Domain Distribution Network). The DDN http://www.domaindistribution.com/index.htm is a network of registrars, secondary sellers, and brokerage houses that have the ability to buy/sell domain in real time. I've had a couple of my domains bought using this system and I can tell you that there is nothing easier. As long as the domain is registered at a network registrar the name can be instantly sold and the funds instantly deposited into your account. To the best of my knowledge the DDN has not released what metrics they use to consider a domain as 'premium'. I was doing research for a name @ GD and found that one of my names was listed as a 'premium' name for sale at GD. I had never listed the name for sale at GD's TDNAM so I was a bit surprised to see it listed. I did some research and found out that GD had joined the DDN. If any of you sell many names or have good domains to sell I would suggest this system. If you have a Fabulous.com account you can just go in and change your account settings to 'Instant Transfer'. I'm not sure how Godaddy and Moniker work.