Might be in your best interest to visit one of the Oakley stores if one near you. (the ability to try and see in hand what they are like) If not, try reaching out to them directly and seeing if they can give you answers on sizing or specifications on certain models.
If you are looking for sellers on Ebay, i would suggest looking at there feedback, what feedback they leave for others, what other items do they sell and look for a authorized dealer of new/used. Some dealers of new will occasionally sell used ones. Ebay does have a good MBG for buyers so make sure you read it and understand how to proceed if needed. Most of the time you can buy direct which can be good (except you pay retail) if you are looking for guaranteed authenticity. Also buying from authorized dealers/manufactures typically includes a warranty which may or may not be available if you purchase Used or from an Un authorized dealer.
-Good Luck
