Curtis only finished a limited number of book sets (about 250?), but a lot of pages were printed that were never bound. Most -- if not all -- of the Curtis images you see on eBay are from those unbound pages, and there were many 1000s of those.
Each volume of those books now goes for something like 30K or more, if they ever hit the market, but these other pages have been in the market for some time now.