I guess I would call myself an amateur Oakley expert, as I have owned over 20 pairs over the last 18 years.  Buying Oakleys on Ebay can be a dicey experience if you aren't careful.  It is really tough though, because what may be pictured in the auction may very well be an authentic pair, while what is sent to you may be a very good forgery.  Here are my tips to avoid Foakleys...

1. Take your time in bidding and check the seller's feedback.  If there are numerous neg's where authenticity is in question, save yourself a future headache and avoid them.
2. Make sure that no stock photos are used in the auction, what you are bidding on should be pictured in the auction
3. If you are buying from a volume seller, make sure the box, bag, warranty card, and the registration card come with them.
4. Once you get your glasses, take them to a Sunglass Hut, or other official retailer and compare them.  Also ask the salesperson their opinion, some can be quite knowledgable.
5. Trust your instincts.

It seems pretty unlikely that you will spot a fake being passed off as the real thing on the auction, so probably the most important thing is to verify the glasses after you receive them.  If they are fakes, don't fly off the handle immediately, be a gentleman...for the moment.  Rationally present your argument to the seller and try to remedy the situation before things escalate.  That is probably good advice for all Ebay transactions.  Hope this helps.