Why not ask the seller?
Ask them to provide you the serial number, or you won't purchase without it.
(Not sure why you would need it, as you can glean something about the Mac from the OTHER information on your example.)
Some folks might cover the serial number, because the logic board has been replaced without using the serial number tool to burn the serial number to the replacement logic board, or the serial number no longer is the same because the logic board was replaced with a used one.
Or, there could be other possible reasons why a seller wouldn't want to provide a serial number - not all of them involving bad news.
I also "dunno" - but the seller knows. Ask him. If they don't have an answer you like, move on.