Since it's legal for me to pay legal tender for software, for jpegs, for mp3s, and other digital constructs, how could a legitimate argument be made to make it illegal for me to pay legal tender for copyright-infringing files?

Since it's legal for me to take legal tender for software, for jpegs, for mp3s, etc., how could a law be passed to make it illegal for me to take legal tender in return for copyright-infringing files?

If I advertise that I want to buy copyright-infringing software, how can they stop me from spending my dollars/euros on them? If I advertise that I have copyright-infringing files to sell, how can the stop me from doing so?

I won't pretend it's not silly, but only so long as you stop pretending that governments have never made illegal the exchange of certain bit patterns.