I found this article on "book myths" very interesting...thought you might, too...enjoy!
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Myth: All books have at least some value.
Reality: Stand in front of a bookcase containing, say, 100 books. Typically one or two of them will have enough resale value to bother with. These are the odds we booksellers face every day. The market's cold reality. Worse, many of the remaining books will have negative value - that is, there will be an expense to transport, store and/or dispose of them. The short reason for this revolting state of affairs is that the online marketplace has too many books and not enough buyers.
Myth: Old books are valuable.
Reality: Human nature doesn't change much. A hundred years ago there was no radio, TV, etc., but there was a strong appetite for the same species of vacant content that viewers lust after today - that is, books were a primary source of light entertainment. Like TV shows, many vintage books were written quickly, had almost nothing of importance to say, and were executed so poorly that nobody in their right mind would want to read them today, let alone collect them. Also, printings were often large - this was especially true of fiction - and with few exceptions these are a tough sell.
Myth: Signed books have value.
Reality: Highly collectible authors (and similarly collectible VIPs, celebrities, etc., who have put pen to books) comprise a very small percentage of the total faces in the marketplace, so the chance of a signed book having significant value is correspondingly small. If an unsigned book has value, the added presence of a signature may get it sold faster, but unless the signature is collectible in its own right, it's unlikely to significantly increase the value of the book.
Myth: First editions are valuable.
Reality: Most first editions have no more value than anything else. The reason is that most books weren't good enough (or in enough demand) to merit additional production and exist only as first editions.
Myth: Bestsellers have value.
Reality: Intuitively, it makes sense that bestsellers would make excellent candidates for resale simply because there's so much demand for them - and yet, if something sold for $29.99 a year ago, it might be difficult to understand why it's worth less than a buck today. The reason is simple: oversupply. Best sellers are printed in huge numbers to satisfy a huge initial demand, and once the primary market has been saturated, there are far fewer potential buyers waiting for copies in the secondary (used) market. Bestsellers may have a modest resale value if the secondary phase hasn't been reached yet - that is, if they're resold in the early months after their release (when new copies are still heavily stocked on shelves in new bookstores).
Myth: Scarce books are valuable.
Reality: Only books that are both scarce AND in demand have value. Again, most scarce (or uncommon) books have little or no value because large printings couldn't be justified at the outset. You may love your grandmother dearly, but that collection of florid, saccharine poetry she self-published at a vanity press will likely have no value whatsoever to anybody outside the immediate family, if that - and yes, even if it's inscribed, dated, numbered and handsomely housed in a presentation clamshell box.
Myth: Big books have value.
Reality: This misperception is especially prevalent among estate liquidators. Drop a coffee table book in their laps, and they'll slap a $10 or $20 price tag on it without blinking. The truth is that, as time marches on, fewer and fewer large books have significant value. One reason for this is that the coffee table phenomenon is a fairly recent publishing development; most big books, therefore, are both newer and more common. Also, many coffee table books, especially those published in the last decade, are often nothing but mass-produced reprints that began their retail lives on those bulkhead discount display tables at Barnes & Noble. (The best indication of coffee-table-book reprint status, by the way, is the absence of a price on the dust jacket.)
Myth: Encyclopedias are valuable.
Reality: This is a hard one to swallow, especially if you dropped a G or more on a set of Britannicas 20 or 30 years ago, but again almost nobody wants them now.
Myth: Otherwise valuable books in bad condition still have significant value.
Reality: Rarely. Condition still ranks as one of the most critical factors in determining value in the marketplace, and no matter how old a book is, collectors still expect the moon and see no reason to forgive poor condition at any age - unless forced into a corner by utter scarcity.
Myth: Big collections are valuable.
Reality: Only if the individual books are. Anybody can amass a large collection of books without lifting a finger - in fact, if you don't jealously guard available shelf space 24/7, shelves will fill up with books as soon as you turn your back. If a collection is specialized, it may have more appeal to the right buyer, but ultimately, it's most likely that 1,000 near worthless westerns have only 1,000 times as much value as one near worthless western.