So the reason most bike shops don’t sell used bikes [I’ve been told] is that you need a pawn license to sell any used products. Apparently the process of getting one is difficult and time consuming, and most bike shops seem to do just fine selling only new bikes. The unfortunate reality is that most Pawn shop bikes are either total garbage or likely stolen. The two suggestions above, either Phoenix Bikes in Arlington or the Courthouse/G-town market, are your best bet. Also possibly craigslist if you have some mechanical/bike knowledge and are willing to walk away from anything shady [also lots of stolen bikes on CL].