This is a pretty typical starter set, $30 from Disney for the lanyard and four pins:
This Pirates Pin Starter Set is a great way to get your Disney pin trading collection off to a buccaneering beginning. The four pins feature popular Disney characters dressed as characters from Pirtates of the Caribbean: On Stranger Tides.
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.