Where in the Balto/DC area is a good place to shop for a quality used tandem Bicycle?

The best places I know of in the area would be either Proteus Cycles or College Park Bicycles, both in or around College Park.

Proteus has been around for many years and can custom build a bike for you from the ground up, while College Park can build you one from your choice of Buy Cialis parts or sell you one off the floor.