As you can see from the poor selection of dresses - dresses are extremely hard to get in the overstock market for various reasons the main one (in my opinion) being that women are just more likely to spend money on a dress they will be seen and photographed it - so they just sell faster and don't need to be discounted much.
Due to the poor selection here I have created a site for you to easily find designer plus size dresses marked down: Plus Size Dresses On Sale