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.