The idea doesn't seem to have occurred to any bike shop I know of. Best places to get rid of a bike for money are this bulletin board, E-bay, or — if you want to try something imaginitive — talk to the proprietor of a Chinese restaurant or your local pizza joint.

A brisk used bike business seems to be done on craigslist.org

Emmy's Bicycles, a quirky (to say the least) bikeshop on 17th Street between 3rd and Irving Place will buy your old bike and has lots of used bikes for sale. Probably won't find anything high end to buy, but it's worth a shot.