Flippa allows you to buy and sell domains, websites, and apps. Buyers can bid up auctions for domains they wan to purchase. Sellers of domains will pay a basic $9 for listing domain for sale and 10% for successful sale of domain. I have been to other domain sale sites but Flippa appears to be the easiest to use with a clean interface.
Check out Flippa HERE to buy or sell your next domain name, website or app