did you follow the suggestions in this thread?
did you set a from email address that corresponds to your server (as mentioned in the big , red DMARC notice in the admin if it is not )?
Did you have a look at the (mail) server (error) logs ?
by default the plugin sends the same email to buyer and seller in the same email (using cc’s etc), if you only receive one of these, the chances are it gets rejected by the mail server for the other because of – perhaps – firewall, MX, PTR, DMARC settings of the server.
you should speak to your mail server admin. they would be much better placed to find out as I have no idea about your setup (mail or otherwise)