My opinion, but I wouldn't list it for less than $300 to start with as it's in running condition and looks in good shape, although the rear cylinder it's prone to failure and can sound like it's running properly. A way to tell is to listen for irregular pops coming from the motor, indicating a failure from one of the cylinders, most likely the rear.
The other way is to feel the handles for excess vibration that causes discomfort to the hands when running. They are renowned for their smooth running, so if it's feels as rough or rougher then an average two stroke, it's not running on both cylinders.
As a guide, one just like yours with the one owner and it's 1981 purchase papers from new at Waltons fetched $650 on the bay of fleas a few years ago. It had it's original dark brown catcher. Not sure if yours is a black replacement catcher you've got there.
Anyway, I'll be interested to hear how you went with the online auction.
