James,
I purchased one in similar condition as the one on Gun Broker for $800. A gun store would offer you a few hundred dollars. If this Parker was handed down through the family it is priceless. Do you shoot waterfowl or sporting clays? If so
http://www.rstshells.com/store/m/1-10-Gauge.aspx these are the only commercial shells available with appropriate length, velocity, and pressure for guns of your Parkers vintage. If you don't shoot just hang it over the mantle as your Great Grandfathers Parker. Again, priceless.
Kindest,
Harry