" I was after a book with an ancestor's diary in it, or part of (No privacy for writing). Not a rare book at all, but one it would be harder to come upon by chance."

Family histories sell reasonably well. Usually very small print runs and may have been published 30 or 40 years ago. An individuals family history is a great example of an uncommon book that people are looking for, but cant find in an op shop or garage sale. You can buy them for a couple of bucks and they sell for $25 - $35 depending on quality and time out of print. They don't march out the door, but are steady, reliable sellers if you are willing to list and wait for the right buyer to come along.

Books don't have to be old to be worth reasonable money. They just have to be rare / small print run ( that people still want ) or highly specialised subject matter.