Add to that, advertising post cards and post cards from someones home town along with a town or post office that is no longer open............... .People collect pc's for many reasons.........Who, what, when, where, why. Most sell for a couple buck to $5 or $10, but some for much more.
You will have to research your cards to receive top dollar. A dealer will take a quick look at your books, base his price on maybe a handful of good cards and price the books accordingly. A dealer has to sit on most of them for some time. Some will never sell and he should offer you about 1/2 of retail.