PoS user informs the PoS Dev of sold keys, before the attacker can launch his attack ,

PoS Dev updates checkpoint thru program update, making the attacker purchase of old keys useless.

Now the attacker has the useless keys and lost his payment  , and the PoS User & Dev are Laughing at the attacker's attempt.
 
*What is funny is the attacker whom is attempting to do harm with a dishonest heart, thinks the PoS User will be honest with him.*  

This might be the primary reason , no one ever tries to buy old keys.  
Because it is so easy to turn the attacker into the Chump.