While not all the information is guaranteed to be 100% accurate, you can look to see what information is available in the message's headers.
This will show you which servers have sent and received the messages. But, the originating server can be faked.
But, to look and see what is there, go to the "View" menu.
Then, select Headers and then "Long Headers".
This will cause Mail to show everything it know about the message.
But, if you forward the message, then mail will not include that header information with the message unless you have those long headers visible before you click the forward button.
So, make sure you have it set to view long headers before you click the forward button. Then, make sure that it shows that full detailed header in your message before you send it to e-bay.
Without that detailed header, they won't be able to tell anything from the message anyway.
Hope this helps.