I am surprised to read this. What kind of systems are these?
I'm not sure. I guess it's some customized wallet software.Quote
How are they able to monitor if a private key is leaked or not.
See for instance Collection of 18.509 found and used Brainwallets, but it could just as well be from hacks or leaks.There is a way that you can make money with them, if they are old keys and moved coins in the past, then you can get clam coins with those keys.
Clams aren't worth much anymore. And I'm pretty sure someone thought of claiming them already 
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And another fun way to make money with that list is by selling it in one of those services that let you sell files or compressed folders for cryptos. For sure some users would be interested in buying that list.
Good point. They're probably being sold already.