¿Where feedback on the trader feedback system will be recorded?
Could you please clarify what your question means? Are you talking about this feedback thread about the system overall, or are you talking about the description of each transaction / trade feedback details?
Also, just to be sure... this is only for trades after the system goes live? That seems the fairest way to do it, but I know that some issues existed before, so wasn't sure...
No, it does not have to be only sales made after the system went live, as stated in the OP. If you have a recent-ish transaction that happened before May 8, 2018, you can still contact your buyer/seller/trade partner and exchange feedback. Many people keep their old sale threads untouched (preferable to those megathreads where the OP gets cleaned out regularly), so it could be easy to reference those older threads in the feedback.