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If you won't have a car, a lot of the places you listed will have extremely limited transportation to DC during rush hour (let alone at random times of the day or night), and a cab would be very very expensive (if they'd even take you that far). Not to mention it would be difficult to do everyday things like grocery shopping.
Your best bet is to go the group house in DC route. Cabs will be easy and relatively inexpensive, you'll be able to walk or use the Metro for errands and entertainment, and have no need for a car.
And yes, be very wary of scams. CraigsList can be great, but if it looks too good to be true, it is.