Since you are in Austin Craigslist is a great resource. I don't know any bike shops that sell much used, but I only go to a few I like.
Bicycle Sport Shop has a fantastic selection although $500 might be close on a road bike but maybe doable, I would check it out.
Austin Tri Cyclist usually has some sort of beginners bikes and have a woman co-owner, might look there.
Jack and Adam's is another good resource, I think I found a Felt when I was looking for a commuter in the $600 price range, so not far off your mark.
Amanda
2011 Specialized Epic Comp 29er | Specialized Phenom | "Marie Laveau"
2007 Cannondale Synapse Carbon Road | Selle Italia Lady Gel Flow | "Miranda"
You don't have to be great to get started, but you do have to get started to be great. -Lee J. Colan