What should I do before selling? Is it as simple as clicking a reset button in the H9 app on my iPad?
No, clicking the reset button in H9 Control resets H9 Control's settings. It doesn't do anything to your H9.
If you want to reset your H9 to its original factory settings for the buyer, deregister it in H9 Control using the deregister function under Pedal Settings, and then turn the H9 off and then on again with the right footswitch and the big, black wheel pressed. That will restore all of the original factory presets.