Paul Collins, Sixpence House
As for theology or religious books of any kind, they do seem to get little attention in secondhand stores or used book sales. Titles by certain authors may be exceptions to the rule. I'm thinking of such people as C.S. Lewis, Dietrich Bonhoeffer and Henri Nouwen.
Celebrity autobiographies don't sell well in secondhand stores because a) they are usually not well written and b) celebrities come and go. Some people might once have been eager to read a book by Pat Boone, for example. But who would be interested now? The same is true of political autobiographies. Even during the political campaign for which a book was written, few people wanted to read them. Later almost nobody does.
It's amazing to me how many books on military history are published each year, so somebody must read them. (I've read quite a few myself.) A shelf of military books in a secondhand store may get little attention, but to a certain reader, probably a man and probably a veteran, it can be considered a goldmine.
Looking at the question from the perspective of a book buyer, rather than a bookseller, I could add other categories to this list. Take former bestsellers, for example. Most people who wanted to read a book like The Bridges of Madison County have already read it. Now there are countless copies of the novel out there and relatively few people interested in buying them. Most people want to read today's bestsellers, not yesterday's. I usually skip over such books very quickly when I am shopping. Sometimes there are exceptions, however. A couple of years ago I bought several secondhand books by Alistair MacLean, once a bestselling author, because I had neglected him at the time but was now interested in seeing what I had missed.
I ignore most self-help books when they are new, so naturally I ignore them when they are used, as well. Again there may be rare exceptions.
Finally there are self-published books. When you have to pay somebody to publish your book, you will probably have to pay somebody to read it.