Generally speaking, the pins you will find on ebay are ones that tend to be limited edition and/or "harder" to get. As someone said, for regular "rack" pins, it's not really worth it for the seller to list them nor for the buyer to bid on them. Unless it's something unique, if you can pick one up off the rack in the park, no one sells/buys it on ebay.
Having said that, you can often pick up these "harder to get" pins for cheap. There are just so many pin auctions, very often you can pick up pins for less than they originally cost. But always factor in the shipping cost! You're best bet is looking for people selling lots of 10 or more as everyone has said.