Fake is an interesting word. Disney gets their pins from jobbers in China. Sometimes, when the contract for a pin is fullfilled additional pins will be made above and beyond the contracted number and then the excess will be sold, primarly on ebay. This becomes a problem for collectors when it involves low number limited editions. It is also a problem when these pins are traded to CM's who don't discriminate and then trade them back to visitors who aren't careful or knowlegable.
Bingo. This is exactly it. So many people think that just because they are stamped on the back, or the seller has good feedback or that they traded with a CM that they are "real." Problem is that they're all "real" in the sense they're all made in the same factory from the same molds.
The difference is that if it's $1/pin, it's not authorized. It's overruns and seconds and things like that. Pieces made above and beyond the authorized run of the pin.
So lets go with unauthorized then. Personally I hate it when people refer to them all as "scrappers" as most of the ones on the market now don't fall into what a true scrapper is. A true scrapper is one that would have normally been tossed for whatever reason. So they would grab them from the trash to resell. Now as the PP's pointed out, many are made in the same factories using the same equipment. They often use a thinner/lighter material, colors may not be perfect, but most people won't notice.
Some are completely fake, and have never been made in any shape or form for any market. One of the reason that most CMs don't care is they have no idea about the pin releases worldwide. WDW alone typically releases dozens of new pins a week. Adding in DLR, Disney Store, Disney Soda Fountain, WDI and CM exclusives add to that count. Never mind the ones that are exclusives to Paris, Tokyo, or Shanghai, Disney Store Japan, or any of a plethora of other legit sources.
Or for instance the Vinylmation pins. I think there are around 400 legit VM pins. But despite that, there are still several lines they have never made official pins out of, and some lines they only partially turned into pins. So it's very easy for a factory to take the art that gets publicly released for one of those non pin VMs, and crank out a fake pin that most people would have no idea was never made.
Some fake (never produced by Disney) pins were shown to the some CMs in charge of pin trading in the US, both VM and regular, and the CM had to go consult with their official books before they could say that they were definitely fake. Some of the fakes are that good.
For "open stock" pins...it's not as big of a deal. It's a bit immoral depending on your take on the issue. For rares/Limited Editions and such, well, it really devalues your pin's rarity.
CM's don't really care and aren't going to do more than a cursory look at a pin and they will take your $1 ebay pins just fine. Odds are, most on their lanyards are also $1 ebay pins someone already traded them.
But if you don't want to be involved in all that, the only 100% sure way to know your pin is authorized is to buy it direct from Disney. Beyond that, it's kinda a crapshoot.
And this is where the fuzzy lines come in. Are you planning on collecting/trading for value, or because you want to enjoy the hunt and like the pin for what it is? If you are trading for value, then you need to know exactly what you are looking for, and I can see you taking offense with the ease of obtaining unauthorized pins. But if your 7 yo son is the one trading, odds are he will just want that cool looking pin, and not have the slightest idea that it is unauthorized.
All that said, I still enjoy pin trading, and notice lots of kids that enjoy it as well. Saw one boy the last trip trade with a CM, and then trade one of his other pins with the same CM for the exact pin he had just traded away. His dad was like "Son, you just traded him that one." And the kid was like "So??" If you do buy any expensive pins, say as a memento of a special event, whatnot, you may want to keep those in the room otherwise they may get traded away very quickly. Heck, my mom spent our last trip trying to complete the villains bowling pin set. Somehow she had never done it before but had alot of fun tracking them down.