I agree with pps - be very careful of certain sellers on ebay that sell fake pins. If the price is too good to be true they are probably fake. Another clue on auctions for fake pins is if they say that you may recieve doubles or triples of pins. Big red flag for me!
You also have a greater chance of getting fakes if you buy large lots of pins and the seller has alot of auctions for big lots. For example if I was selling off our personal collection I would list the more valuable pins separately and then list the rest in lots of 25 or 50 pins, no doubles (as most normal collectors don't usually keep their doubles or get doubles to begin with), and I would only have 4-6 large lots available for a limited time. On the other hand, a seller selling fake pins is going to have 10-20 lots of 50 pins available almost all of the time indefinitely. I would be more leary of buying pins from someone who has nothing but pins for sale and huge feedback numbers --- red flags for me is how can they get so many pins and sell them for so cheap? B/c they're fakes.
Also look at where the auction is located or shipping from. If the auction is shipping from Hong Kong or China, FAKES! If the auction doesn't list its location, look at other auctions that seller has going on and chances are in the fine print, you'll see item ships from another country. Also if a seller is based anywhere not in FL or CA, chances are they are selling fakes. If they are not in close proximity to a Disney park, how are they able to go to parks to trades for hundreds of pins???
These are just a few of the red flags I have noticed for sellers on ebay selling fake pins. Buyer beware.