I second Belmont Wheelworks; they have a whole store dedicated to used bikes. You would get fit properly and have a range of choices.
Also, Quad Cycles in Arlington may have some used bikes. That's where I got my new Jamis. They seemed to have a big selection of lots of different types of bikes.
When we bought our Kuotas the shop cleaned up our Treks and sold them on E Bay very quickly. We got the money. It seems like my WSD sold within minutes of it being put up there.