Long Time Passing

Children's Picturebook Collecting offers the following statement:

Based upon our experience, there are fewer key collectible picturebooks on the online market than a year ago, continuing a trend we have seen over the past couple of years.

Try a search on any of the metasearch book finding websites, such as ABEBooks, Addall, Bookfinder, or the ABAA, for first edition Caldecott Medal books, or Beginner Books, or I Can Read Books, or Seuss books. Sort the results from high price to low price (the thought being the high priced books would most likely be first editions), and see how many books turn up. The results will show that many first edition books are not currently being offered for sale.

Entirely possible. They go one to explain why this might be. Well worth a glance by your eyes, if you like.