Power
Oh sure, rare book dealers have tons of power...over rare books. That's about it, though. Maybe that doesn't sound all that impressive, but to a person who's obsessed with old books, this is a power worth having.
As a collective, rare book dealers have a lot of control over the prices of their hard-to-find tomes. They're the supply side of a leather-bound niche of the supply and demand economy, and in this world they're a big deal.
Of course, if a rare book dealer sets a crazy-high price that not even the most obsessive collector will pay, then that dealer will find themselves out of luck. So, if you look at it that way, they're still very much beholden to the power of the consumer.
Another thing to think about is that rare book dealers are usually their own bosses, which gives them a lot of personal power in their everyday lives. If they don't feel like dealing with any pesky customers, they don't have to.
They can just spend the day binge-watching Downton Abbey (which we're pretty sure every rare book dealer loves). Of course, this is probably not the best idea, business-wise. Being lazy and self-employed is usually a bad combination.