Cpl100, there are many people who search by category. I've learned this by watching my Andale counters. If a book is not getting many hits in one category, I'll switch it to another category, and will sometimes see a big upswing in hits. A lot of collectors are real fanatics and want to look at EVERYTHING related to whatever they collect. For instance, I love old medical books and implements and will often browse the medical collectibles categories.
That said, listing in two categories makes Ebay fees add up really fast, so I don't do it that often, generally just with items that I think have a lot of collector interest and will go for a good sum.