bc,
Your amp sounds like it is in excellent condition.
With the hum fixed, it should be worth about $300 to $350.
You never know what it will actually sell for – on ebay sometimes Kustoms sell for more than they are worth and sometimes for less.
Is the hum a low level hum or is it fairly loud? You shouldn’t be able to hear much or any hum when you are playing at a low to medium volume. Does the ground reverse switch help any? You might want to try plugging the amp into different outlets – that might make a difference. If you were going to keep it, you should change the cord to a 3 prong anyway, and it is possible that this might reduce the hum.
-LesS