I just discovered this online article and though it was written in November 2005, the general sentiment and concerns remain valid. Of special note was the comment re collectible Civil War books:
"For example, a collection of Civil War books might have been worth $200,000 some 10 years ago. Now because of the Internet, the collector would be lucky to get one-fifth of that," Baldwin said. "People find some books in their attic, do not know the value of the volumes and put the collection online. Without professional appraisals, the collections are undervalued, which drives the price down for everyone," he said.
Not just ACW books, but titles in every possible subject and genre across the board. As discussed in an earlier post, supply has opened up for every collector, thereby pushing down price.