I've tried this in both Chrome and Firefox (both updated to newest versions), and am getting the same result. When on the "post new auction" screen, clicking on an item to post results in a partial auction-posting window. I can't modify the price per item, and can see the item icons through the window.
This is difficult to describe, so I've posted a screenshot here.
As I said, I can't modify the price of the item or the quantity. The X to close the window also doesn't work. I'm not sure about the "post" button.