'Designer' is such a pointlessly nebulous term. For the sake of the discussion lets assume that by 'designer' you mean sunglasses with the name of some designer on them like Lauren or Armani or Tom Ford. Do you really think these designers sit down at a drafting table and have even an iota of Input in the formulation of their designs? And whether you answer yes or no, would you rather wear sunglasses designed by a designer who works for a company with some history of optical know how or would you rather wear sunglasses designed by someone who works for a company that designs suits and dresses?
Buy sunglasses from optical companies. Buy watches from watch companies. Buy suits and shirts, of you will, from 'designer' companies. eBay is always a crap shoot but there are some clues to whether sunglasses are real or fake. From other sites, I've bought from www.eyesave.com
(prices are better than retail and they occasionally tease you with another 15% off). If you're in the mood for traditional aviators you can't do better for price and quality than Randolph engineering. As for the rest, bear in mind that of it seems to good to be true it probably is. Good luck.