Well, yes of course, there are loads and loads of fake designer sunglasses on sales over the Internet. Since they are generally very cheap and customers could not resist buying them from time to time. So, my advice for you is to check the serial numbers which are designed to tell the authenticity. Then ask for any warrenty or certificate that could prove it it real. Or you might as well contact their customer service dept to ask for help.
Well, you can surf the one that you like on other sites. Finding more pictures, and compare them with the one you chose on ebay. Many customers who bought sunglasses on ebay said that look over the brand and the size of the word. The fake are much bigger than the real. And pay attention to the side of the brand. Some are actually on left, but they are on right indeed. There are so much cheap stuff on ebay that not all people believe they are real. Actually you can buy real and great goods at a deep discount on ebay. Here is a website that can help you know more skills to spot fake sunglasses on ebay.
If you decide to buy the sunglasses from the online stores, you'd better see whether it is real. You could see whether the information about it on the web page is full. If not, there may means that it is fake. If the information is full, you should check it one by one. You could see the original place which may easily be checked. Then you see other quality standards. From the details, you may find whether it is real.