Stress
We're going to go out on a limb and say that being a rare book dealer is super low on the stress scale. Of course, if you happen to be a really awful rare book dealer who sank your life savings into a bunch of dry-rotted tomes that nobody wants, that'll be majorly stressful.
In general, though, your days will be spent hawking your wares at book fairs, chilling at your cool shop, or emailing and chatting with customers at your leisure. Most people who go into rare book dealing are looking for a slower, more leisurely way of life, and rare book dealing doesn't disappoint in that regard.