That SLW is very black most have more white on their backs if they are roos. If it isn't crowing at you I would post it's picture to the Wyandotte thread over here before you take it to the auction.
https://www.backyardchickens.com/forum/viewtopic.php?id=279470
https://www.backyardchickens.com/forum/viewtopic.php?id=279470
