Sedo is a well known site for selling domains. Domain flipping in general is down from a few years ago.
Buying domains in the hope of selling them for good money might have been a worthwhile pursuit a few years ago, but a contracting market means more people looking for a specific domain than a ready built site these days.
Like anything, a unique proposition would make you the exception. I know some who make regular monthly income selling (renting out) sub-directories with a one page announcement/profile for community businesses and organizations.