This is a topic that comes up often on forums, blogs… then me also I conducted my little investigation… In our wedding shop, we often have people who come to us in disaster because they bought their dress on the internet and are sometimes a few days of marriage with a cloth, a disguise see anything… This happens both on the wedding dress than the bridesmaids. Where I will emphasize these are on China wedding sites that offer prices very very attractive but at what price!
So, I will give you some tips to recognize if a website is reliable or not, to buy her dress in being notified. But also the risks that you take and how to minimize them.

Recognize the origin of an internet site:
It is easy to see whether a site is abroad.
French merchant has an obligation to General conditions of sale (t & c) and the imprint of the company even if he is an autoentreprenor. Click on it, if you find an address is that the site is transparent about its origins. If not it is fishy and the site wants erode you input about his identity… and that it is not french…
Go directly at the bottom of the site and look at the logos of payment and delivery. Chinese websites have a form of payment by paypal with credit card logos put forward and delivery often by DHL.

A french site will offer you a method of payment linked to a French Bank and modes of delivery such as colissimo, chronopost, global relay… in short anyone you know on a daily basis.
Another concern for size, Chinese sites in place market as Dressesmallau. This site is a marketplace like Amazon in France where you have thousands of sellers. So it is a little Russian roulette, the site is well known but the seller has that you bought may be not… so it will be as in the good pile.