Lost Redelivered Mail?

Hey so a while ago I dropped off this photocard using Mercari's first class envelope label, but it was suddenly returned to me. I contacted the buyer immediately and they replied saying the mail never arrived and Mercari said they will take care of it. So a few days later i believe, I received the mail as it was sent back to me. There was a little yellow label saying "return to sender, not deliverable as addressed, unable to forward."

Confused, I went back to the post office to have it be redelivered after confirming the address again with the buyer. This was definitely my bad because I should have looked at the Mercari FAQ before acting on my own. I didn't realize Mercari labels could only be used once and you were supposed to cancel the transaction first before having it be redelivered.

This was not the first issue I had with their first class envelope label. Before I had one returned to me because of insignificant postage..? But i reweighed it and it was fine so I'm not too sure what went wrong.

I am assuming since the label could only be used once, the tracking for the label doesn't work anymore :( I'm not sure what I should do now 😭 I'm stressed out about the fact that it is lost. Should I cancel the transaction now..? Or would it be too late. Is there anyway I could find out where the mail went?