Figure it's worth half what you paid, the second that you walk out the door with it. And that's if you're LUCKY!
Very much like custom/restored cars or boats, it's far, far better to let someone else take the initial hit for the restoration $.
And it's been my experience and observation that auction houses are places to avoid like the plague. Even if they're honest (a very few actually ARE), their charges are ridiculously high IMO and they frequently are wrong on their prices and descriptions.
Plenty of good photos and a complete & knowledgeable description will go a long way toward selling it on Gunbroker or one of the internet talk forums.
Regards, Joe