Use a reputable seller like best buy, newegg or toysrus. That way you have a way to deal with customer service if something is wrong.
Good tip, however none of them sell DualShock 3 new. And no local stores have them, so stuck having to trust some seller on eBay.
Unless it's from a major retail seller, assume every DS3 on ebay is a knockoff. There are a ton. They can look identical but the weight and feel is where you'll notice the difference
No major retailer is carrying new PS3 accessories anymore. :(