They give a link in there that I haven't seen before, for searching a hard-to-find book for sale:
bookfinder4u.com
I was relieved to find that a volume I spent a considerable sum on when someone finally offered it for sale at a price I could muster is currently available from only five sellers, and their price ranges from 50% higher to 5 times the price I paid. Patting myself on the back.
Looks potentially like a valuable addition to my book-hunting toolbox.