The biggest negative for me about your Dart is the Rot in the Rocker Panel shown in the picture. Many people will consider the car to need a fairly complete restoration. Not trying to put it down but that's probably at least part of the problem. It's a beautiful car though.
If you were to list it on Ebay with No reserve it would sell. You'd get tons of hits and you would get a pretty true number as to what someone is willing to pay for it.