A domainer is, in domain names lingo, an individual who makes a living - and for some a very good living - of registering, buying and selling domain names. A domainer can also generates revenues through domain parking.
In the beginning of the Internet era, domainers registered domain names with one or two words matching generic words and terms and occasionally resold them with a spectacular margin. Some generic domains originally registered for less than $50, have been sold for several millions of dollars (see domain name buying for examples).
Rapidly, all domains with one or two words have been registered. Now, domainers who are still buying on the primary market (domain registering) buy domains with several words or domains associated with new trends, new words or new domain extensions (see the interview below). They also may buy domains for which registration is not renewed.
Domainers may also have a domain trading (buying for reselling) activity by which they buy a domain, then resell it with a margin after searching for potential buyers.
Some domainers buy and sell few domains of high value and others achieve hundreds or thousands of transactions with smaller margins. These latter ones use automation for buying and selling domains.
Domain may be sold through one-on-one agreement or auctions (see domain name auction for more details and a specatuclar video)
A very interesting interview with a domainer: