www.elderly.com
They have a form that you fill out on-line and email it to them, they will send you an appraisal. They are usually right on.
They always have many vintage Martins and I think they take 15% if you consign with them........
http://elderly.com/brand/MARTIN.html
Here is the consigning evaluation form.....
http://elderly.com/cgi-bin/elderly/evalform.pl
I have bought two new martins and two new PRS guitars from Elderly. They have the best staff that I know about. Great people.