You bought something you knew was a "final sale" and you allowed yourself to succumb to high pressure sales tactics. You've got no legal argument I can see.
But you do have some leverage here. There's social media such as Facebook and Yelp at your disposal, and this store will not want the bad publicity of an unhappy customer, so you might get somewhere with your refund request if you note the fact that apparently according to this sales associate, there was another customer who wanted this dress, and you wouldn't want to have to publicize this unfortunate incident to your vast network on FB, Yelp, etc., so isn't there some way the store can refund your money?
Also, if you paid with a credit card, you can dispute the purchase with them/ Follow the directions on your credit card statement to use this method of dispute, which may only be temporary, but will tell the store you're serious.
Avvo doesn't pay us for these responses, and I'm not your lawyer just because I answer this question or respond to any follow-up comments. If you want to hire me, please contact me. Otherwise, please don't expect a further response. We need an actual written agreement to form an attorney-client relationship. I'm only licensed in CA and you shouldn't rely on this answer, since each state has different laws, each situation is fact specific, and it's impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue.