You should be looking for authentic Disney pins. If you want to be certain your pins are authentic it would be wise to buy from the
Disney Store or
www.disneyshopping.com or from the Disney Outlet if you have one close to you.
If you want to attempt to buy pins from eBay you might want to stay away from "cast lanyard" and "Hidden Mickey" pins because there are many counterfeits and it is hard to determine authenticity. Look to see how many of the same pins the seller has, and look at their completed auctions. If they are selling the same 10 pins over and over again I would NOT buy from them. Sellers buy the counterfeit pins from China by the hundreds, then break them up into sets of 10 or 20 for eBay auctions. PinPics has some good info on avoiding scrappers and counterfeits, and also has tips on spotting a fake AFTER you receive your pins. Never buy from a seller who won't take the pins back. Remember that feedback is NOT an accurate indication of whether or not a seller has authentic pins. There are more buyers who can't spot a fake than can, and they will post positive feedback. Once feedback is posted it is permanent. I bought a lot of 80 pins from one seller and a lot of 40 from another and the majority of both lots were fakes. By the time I realized they were fake I had already posted positive feedback because the pins were shipped quickly and were the same pins pictured in the auction.
Now I know better.
Places that sell pins online for super-cheap prices may be doing something questionable, like selling factory rejects. Sedesma pins are lower quality pins that some people have said they have had trouble trading. ProPins and Euro Disney pins are sort of in that same camp - they are probably TECHNICALLY allowed to be traded but some cast members may frown on them.