Has anyone ever ran into another "domainer" in their every day life?
I tried once, but he managed to jump back on the sidewalk in time.
OK, seriously…
As redemo has now figured out, 90% of the question is "what's the definition of the word domainer?" To which the answer is: there isn't one (there are lots of definitions).
I can think of two acquaintances who hold domains that they would like to monetize. One has two (I think) domains from a business venture that never materialized, but he has hung on to them anyway because he thinks they might be valuable. The other has about thirty because she thinks they're cool and, again, might be valuable. Neither of them has ever sold a domain, or even tried to, and both earn a living in other ways (employment and self-employment). But by a very broad definition, they might be called domainers.
Look at the number of domains available for sale on Sedo, then look at the number of sales. It's tiny. A sell-through rate of 2% is often quoted, meaning that statistically, a "domainer" sells 2% of their inventory every year. For major players, that might be a realistic way of looking at the statistics. But my hunch is that of the people offering domains for sale e.g. on Sedo, a great proportion of them never sell anything. And if we consider how many domain registrations aren't renewed after the first year, it's quite conceivable that at any given point in time, a huge number of people have only recently got into "domaining", and will quit after a year or two after having had zero success.
It's just a hunch, I admit.
But even if I'm wrong about most "domainers" actually being "would-be domainers", I'm convinced that the number of people actually actually make a living from it is only a tiny fraction of those offering domains for sale.
We could just as well ask: What's a poker player?