It's just as ethical as being a stock or real estate broker.
Funny that you mention that, as I am presently studying for my license in Real Estate, and thought the same thing.
I believe that a market value should apply to a domain based on its quality (i.e. length, combination of words, easy to remember, etc.), and in turn compared to other domains that have sold in the past... but that being said, many reject the concept of a market value in the way of domains (hence all the overpriced names on eBay, Sedo, etc.).
What many of us tend to justify a valuation on is the Value in Use... once the potential worth of a domain becomes increased as a result of a discovery of its uniqueness to the end user, it isn't uncommon for somebody to try and gouge said end user for it.
To answer the topic... I certainly think this business is ethical, and hence my reply, I think it's much like real estate (just virtual), but those who trade in real estate must be licensed to do so... the same should be the case for domains. It's too serious a business to leave unregulated.