People that have ebay stores are able to sell items at a cheaper price than their brick and mortar counterparts. A guy selling out of his house does not have to pay for 100 employees and building rent costs. I have heard of people selling fakes, but it is hard to tell who is legit or not. Take a look at the seller's feedback to see whether it is positive or not.
I purchased some Oakley's. No local place would do anything with them. I had to contact Oakley to send them out to get fixed. Other than that, it has been a great product.