Sedo -> Buying, Selling, Auctioning and Apraising domain names - In other words the Ebay of domain names but a bit more.
Sedo offers our users all the tools needed to buy and sell domains among a community of users stretching around the world, including domain appraisals, brokerage services, promotion and last, but not least, Sedo's popular domain parking program. More than 1.5 million domains use Sedo's parking programs to earn revenue while promoting its sale.
Check them out at http://www.sedo.com
GDNX -> This is an idea that not really new, buying and selling domain names but these guys have reinvented this model to operate this company as a financial exchange.
The GDNX operates like a financial Stock Exchange with registrars acting as members on behalf of their customers or registrants. Only GDNX members have the opportunity to participate in the Exchange.
Members can list domain names of any Top Level Domain (TLD) on the Exchange. They can also benefit from access to the complete inventory of domain names listed for sale. Domain names can be listed for sale through a complete API, which includes the functionality to list, bid and buy domain names.
Members have the choice to use the GDNX to list their own domain names and/or their clients' domain names. Domain names are listed through an application interface, which each member can customize to their own preference.
The key word here is that only registrars can be part of this exchange and they can trade domain names on behalf of their customers.Check them out at http://www.gdnx.org/