Private message rules/guidelines?

Background: Doing a trade and user requests PM. I copy paste our Discord conversation. After successfully completing the trade, that user says I could receive I'm doing PMs wrong and that I could receive a TWC for confirming my identity in the way I have.

My question is two fold.

1. Are there any rules to PMs? Or should it be to the satisfaction of both parties?
2. If there are rules, are there any punishments for not following them?

Thank you all for being an amazing community!