I found that Craigslist has a good amount of road bike sellers. The sellers I've come across are selling their bikes because they have upgraded to full carbon.
I was able to get my first road bike (2010 Specialized Allez Sport Comp) for $550 several weeks ago from a local seller on Craigslist. Excellent condition, only 200 miles on it.
From doing a quick Google search on the bike you speak of, 2010 Giant Defy 2, it looks like it goes for in the $500 range. So, it's not a bad deal I would say.