One of the biggest challenges online fashion brands have to overcome is fitting. If you've worked in retail and had to clean up the dressing rooms, you know the high proportion of items that are rejected after trying them on. I buy most of my clothes online but I also return a lot because they don't fit. There's going to be a lot of returns. Zappos shoes might be a good model - how much of their margin is given up in the cost of free return shipping?