I'd say yes as the Seasonic X650W brand new is $125. All I would do though, is ask the seller how heavily it has been used as this will be a big factor affecting the PSU quality. Remeber if it seems too good to be true, then it probably is
it still a pretty good deal, but ask him how much load he put on it. it can affect how efficient the caps can be and how much actual wattage it can supply as psus degrade overtime from usage