Buying used still takes away supply that is filled up with new purchases that profit JKR, resulting her using your money kill trans people.

Piracy still increases sales because the IP stays in the zeitgeist. At a personal level, this means that if you talk with others about having enjoyed pirated HP IP, you are stochastically advertising for JKR, resulting in you giving JKR that person's money that she then uses to kill trans people.

"No ethical consumption under capitalism" still applies, though. It's often impossible to avoid contributing to murder. As long as you are trying to undermine capitalism, it is okay to keep yourself happy using whatever means available to you that you can stomach.