I ordered a cheap lot off of ebay--it was a mixed bag as to whether they were the real, official ones, or knock offs. In some cases, we didn't care if they were knockoffs, the kids kept several anyway--sometimes it's a very minor difference and they still look fine. That's down to personal preference, as long as you're not trying to pass them off online as originals.
As far as trading in the park goes, however, they will accept just about any trade you give them. They don't inspect them or anything, and they know that people are trading fakes and fantasy pins (made by fans or other companies than Disney). So there will be fakes up for grabs as well. You get pretty good at spotting them. For example, the Disney ones tend to be smoother on the surface, like the enamel is completely filled right up to the metal outline, where as the knock offs tend to have a slight dip in the enamel below the outline of silver or gold metal--the coloration tends to be just slightly off, too. Again, for the most part it's a minor difference, and if you really like a pin, you might as well grab it whether it's the real deal or not. And some of the fantasy pins are super high quality and beautiful, so I wouldn't care it wasn't "official." However some of the knockoffs are clearly inferior. But Disney will still take them and trade them in and out, so just keep an eye on the ones you take to make sure you really want them.
