In Merc you should be able to send the buyer a message on the actual item that you sold. Go to that listing and under the buyer section there is a spot to send a message. These messages are only seen by you and the buyer.
Thank you for that info.
I can't find my original listing. I reactivated the old one and now there is nothing on there about the buyer. My profile says I have 2 listings but only one is showing up and it's the reactivated one. The only place I see that references any information on the buyer is the message from Mercari saying they cancelled my listing. What a pain.

I wonder if I inadvertently deleted my original listing.