I have always had migraines and dealt with light sensitivity my whole life.  So through the last 30 odd years, I have had my share of experience with sunglasses.  I've had everything from dollar store pairs when I was a kid to major luxury brands after I established my career.  Sunglasses are like my American Express Card...I never leave home without it.  The brand which I am highly impressed by and finally settled into has been Oakley, with my personal collection approaching 30 pairs,

With Oakley of yester-years, you were really limited to the sport specific look that is just too out of place in a professional/formal environment.  However, in recent years, Oakley offers a line for just about everything.  And their popularity seems to grow as people get a chance to see their quality, both in design and function.  And as with anything fashion that gets popular, someone will make copies/fakes!  And that means someone will get scammed.

I have a large collection and I can say I'm pretty good at distinguishing the genuine article.  Though it took being scammed a few years back to get me to build my knowledge.

The Plastic framed models are most often copied/faked.  Nowadays, Oakley uses metal icons on almost everything so that's a good indication that they are real.  Look carefully at the icon on the earsetems.  They should be set evenly and colored evenly.  Now that's easy on Square O like Gascan, Hijinx, etc. but on Wires, it may take a little closer look.  Oakley has done a lot to show the authenticity of their products.  The name is usually molded into the earstem.  Always test the rubber(obtonium) if applicable.  They should be somewhat soft and "sticky."  And the nose pieces should have a similar feel.  The plastic all around should be smooth and seemless in both shape and color(s).  And the lense should fit smoothly in the frame without popping out with slight flex of the frame.  A little flex is OK, but the glasses should still feel solid.  

On Wires, The model name is always etched on the inside of the stems.  Lower end Wires are often replicated due to their popularity/affordability.  The new Nanowire lines are a lot harder for cheap counterfeitors to replicate because of the complexity in stem material and lens coatings.  My "scammed" experience was a nice looking pair of Oakley wires that had the name scrape right off with a little fingernail pressure.  Next, after a few days of wear, the tinting of the metal on the stems started fading.  A thing that will never happen with Genuine Oakleys. 

Womens eyewear should come with a white hardcase and a white micro bag, unless it is a special/limited/or sports edition.  All that should be available at the sale to garauntee authenticity.  Many uses 2 or more colors in the frame, and on those, the area where the colors meet should be seemlessly smooth.  Icons should be smoothly embedded in the frame and metal.  The model name should be etched or molded in the earstem to withstand scratches.  The finish on them should be perfect and fingerprints should wipe right off with minimal effort. 

I finally settled on Oakleys because of their lens visual acuity.  Their optical quality is the highest among my many years of experimenting with many brands.  Genuine Oakley lenses should have clear vision all around the curve.  The color should be all even; unless they are Gradient in which case, transition should be smooth.  Polarized lenses should have the word etched in the lower left side of the left lens.  Again, etched so they would not wipe off when cleaning your lenses.  Oakley, never puts a sticker on their lens.  I've seen people selling them with that little dollar store "UV protection" "polarized" , magnetic sticker on the lens.  Blatant sign of FAKE...

When buying used, One should never buy a pair that does not come with everything.  Best bet is from people who have all the paperwork, box, micro bag, original case(if applicable).  Those tends to be the people who takes care of their eyewear, like me, and more likely to get you a nice pair.  Detailed pics should be available upon request.  And they should be easy to contact.  Sellers should have good knowledge of the product from frame material to light transmission of the lenses.  Don't be afraid to ask questions.  The ad should have pics of the actual item being offered and clear details of it.  Stock photos and writeups are easy to lift from the website.  Go with your gut instinct if the offer looks too good to be true. 

Calculate in shipping fees as it often cover the sellers Paypal fees.  Glasses should not cost more than $10 insured to ship anywhere in the US though some people charge you for the packaging and materials.  I've seen people sell for below list but after shipping would cost you ore than if you go to the store.  You are on eBay to get a deal after all.