3 Days of the Condor

[pbr2000]

How do people tell when titles like 3 Days of the Condor are going out of print? Within a day of it going out of print, third party prices instantly shot up to $60.

By contrast, the BD of Casablanca seemed to go out of print (Amazon stopped selling it directly), but 3rd party prices remained low. A few days later, Amazon again started selling the BD of Casablanca.

How did all the sellers distinguish between a title that was only temporarily out of print (Casablanca), knowing not to raise the price, and a title like 3 Days of the Condor, knowing that it was "permanently" out of print and instantly tripling the price?

Is there a way to be notified when a title is about to go out of print?