The image quality differences between these two lenses is actually not too definitive, but most people agree that the 17-85mm does not deliver image quality proportional to its retail price, while the 18-55mm IS delivers better image quality than you would expect for what it costs.
The 17-85mm's main advantage over the 18-55mm is the extra 30mm on the long end and improved build quality. If build quality is not at all an issue, then on the used market you are paying roughly a $200 premium for extra reach, a more usable focus ring, a distance meter, and much more weight.
I personally do not think the 17-85's qualities are worth the extra $200 over the 18-55 IS. There are also some excellent 3rd party alternatives such as the Tamron 17-50mm f/2.8 in the same price range as the 17-85, so you do have some choices in balancing build quality, features, image quality, and price.