I am no ebay expert, but I would say you could probably start the price at 250 or so? That dress sold out of all stores in LA in no time at all, so you might have a lot of people willing to pay a lot for it?
You bought this one off ebay, right? If so, keep in mind that bidders (if they are savvy) will look through your feedback and see that you got it off ebay to begin with ... and they may even be able to see how much you paid.
So, I would start with what you paid for it. If you start there, you'll probably end up getting more than you paid for it anyway.