When buying anything Oakley do your research especially if you are buying from Ebay or off the street. If they are using images found on Oakley's site its no better than buying something with no image at all. Make sure you see detail images of the item your buying. if it has a box IT HAS A SERIAL NUMBER! - Ask for it! If it doesnt well that alone should tell you something. Also you can verify any serial number with Oakley.com or Oakley customer service. Oakleys are expensive for their optics. They are perfectly clear on any axis of the lens. There should not be ANY blurry spots or areas of the curvicture of the lens. This is hard to judge buying online but when you get them check them. Also get the part number for what your buying and match it to Oakley.com or call Oakley. The guys and girls there are quite helpful and you would be amazed at the customer service on getting something fixed.
Lastly if its "Rare" it should have a serial number on the eyepiece. Outside of some new designer pieces they generally fall into this catagory like X-metal and the watches. Outside of Sportsmarketing Editions (Not Available to Public only Professional Sports Athletes) chances are if you see an odd color or diff than what your fimiliar with your best bet is its fake. Custom Oakleys are not "Custom to the Buyer" What custom means is you pick the color frame and color lens match up nothing more. They have not nor will they spend time coloring frames and lense to what ever people ask. NO IT WONT HAPPEN! So again if you see it chances are its fake. I am amazed at after all these years I still see people walkign around with random colors of glasses that I know never existed. If you have a question ask Oakley.
Oakley Display cases became a huge factor for Oakley during 2003-2007 for their retail stores and authorized distributors. Oakley did not sell them to the public for this sole reason of people buying the cases to sell foakleys... "Fake Oakleys". I have seen people go as far as purchase the box and oakley microfiber bag and create a fake sunglass and sell them as real. So you have to be careful when you purchase. Check the seller to make sure they have good feedback and even look for past Oakley Items sold by them that alone is a good indicator.
Unfortunatly there will always be Foakleys around use your best judgement and if it sounds tooooo good to be true remember its Oakley they are expensive but also knowing this info will help you jump on a good deal somewhere.