The title is pretty self explanatory.
The price for them is $0.5 in BTC/ETH/BCH per 1 million private keys.
Why would someone buy private keys when they can generate as many as they want? Besides, what is the use of your private keys for anyone if you have access to them, and if you sell them multiple times?
Some people do not have the computational power to generate and store millions of them efficiently.
Also, no need to resell the same ones over and over again, as I can generate 500 million of them in around half a day.