There is no appreciable difference between the 580, 581, and 582 series Minis. Some early (as in 2005 production only...) 580s still had the "pencil barrel," but I highly doubt yours is one of them. I purchased my 580 in 2008 and have had absolutely zero issues.
CDNN and other retailers buy from distributors with gigantic warehouses. That could be one of the last 580s they had. True, Ruger's up to producing the 582 series now, but I wouldn't be the least bit concerned about that rifle.
CDNN is a reputable place, and any inclination that they sold you a used rifle at new price is absurd, IMO.