I'm a big fan of the $100 beater bike model for getting from A to B, running errands, etc. etc. I have a $100 road bike; one of my roommates has a hybrid he got for even less, and I think it's great to have something that does a job perfectly well and that I can leave locked outside. I can also do stuff like mount a rack and a set of full fenders on it and keep my "good" bikes set up in a more dedicated sporting trim.
When I shopped for a beater bike this time, I ended up buying one from a guy down in Tukwila. I shopped for it on Craig's List, looking for frames close to my preferred size, because the trip to Tukwila was enough of a pain that I didn't want to have to make multiple trips and test-ride multiple bikes if I could avoid it. I wouldn't recommend shopping on Craig's List to someone who doesn't know their size for that reason. To my mind, that person needs to ride a bunch of different bikes and choose their favorite. That means a shop, or at least someone who buys and sells a lot of used bikes.
Unfortunately, shopping for a used bike anywhere is problematic. There are a few places here in Seattle, though, so maybe I'll make a list for her and let that be my evangelism for bike commuting for the month.