I really wouldn't buy off ebay in all honesty. I don't want to start an argument or anything but I think the previous poster has given terrible advice.
There are people who don't care if they take scrappers/fakes to trade for genuine pins but it's really a crappy thing to do as plenty of people have paid full price and receive in exchange fakes. Doing this unknowingly is one thing but being well aware this is likely - pretty rough thing to do.
If you do receive a scrapper/fake in a trade with a CM you can to take to it to customer services and they will help you out. Disney are trying to get rid of these as much as possible but the more people that take them, the more they flood the market.
My husband collects pins and I bought many from ebay before we understood exactly how to identify scrappers etc.
A good idea is joining:
http://gallery.pinpics.com/cgi-bin/browse.cgi
This helps you learn to identify real pins from fakes and learn a little more.