ok but you don't tell anything about my suggestion.
Why not let every user 1.000 listings free every month and take fee only from 1.001st listing ?
In this case you would reach the goal to stop bots update card prices Thousands of times a day without affect the normal user who list a few cards.
As I told I am even fine if the first 1.000 in a month are free and from 1.001st it cost 5 dec.