If dealers acquire a hundred books, they make their money on six and shelve the rest.  The remainder is in most cases going to lose value.  Many dealers understand this and are reducing prices to reflect what they see online.  This helps but few expect this inventory to change hands soon.  There is simply too much of it and more are posted every day.  It seems likely this imbalance will not be resolved until prices drop enough to induce buyers to step in.  Until then the market will have a sell-side bias. 

One way to reduce inventory risk is to emphasize more valuable material and many dealers are doing this.  But given that dealers may hold inventory for five or ten years, the possibility for protracted decline, even among the materials thought to be strong, represents significant risk.  The response of some:  shorten inventory.  It makes sense, but if practiced widely wipes out the sell-back-to-dealers strategy that has sustained the market for lower value collectible books for three generations.  There may simply be too much risk to pay much anymore.

We do know something about why the market is weakening.

[1] We have never experienced the surplus of material we are seeing today.  The listing sites are packed and more material, stored for decades, stuffed into attics and basements, is coming out.  It turns out that belief in the value of books is an old and widely held idea.  No one threw books away and now they are here to haunt us.

[2] The content of many old books is now on line in Google Books.  Did you want the book or its content?  If its the content its much easier [and free] to run full text searches on line.  You can of course then try to locate a copy but more often than not Google Books seems to answer questions and dampen interest in actual copies.