This is a pretty typical starter set, $30 from Disney for the lanyard and four pins:
Have a Pin-Tastic time and collect your favorite character pins for Disney pin trading. Check out the brand new collection with new pins dropping every Tuesday at Disney Store.
www.shopdisney.com
The pins are made similar to other fakes that come out of China, like sports jerseys and handbags. They're made in the same factories as the real ones, with the local (corrupt) plant manager keeping the factory open an extra shift and producing the same products with inferior materials. They're referred to as "scrapper" pins by the pin trading community.
Those eBay lots are typically a mix of genuine pins, fake pins, and damaged pins. I think they're actually worth the price, just use your judgment when going through them. Pins with any kind of texture, movement, or dimension are usually authentic, since they're not worth the cost to counterfeit. Look at the colors, the lines, the printing on the back, etc. Use your judgment and only trade one with a cast member if you're confident that a guest who received that pin later on wouldn't feel ripped off.