The only foolproof way to avoid getting fakes is to buy them, from the racks, at a Disney Store/park. That said, fakes aren't necessarily the best term for the knockoffs. Some are errors, over runs, etc. Others are made on the real equipment, just off the books. IE WDW orders 10,000, the factory makes 15,000 and sells 5,000 on their own. Some are made by other factories using faked dies, and yet others are completely unauthorized, never actually made as pins, but the factory may have taken the art from a sticker, or something else and made pins of it.
Buying from eBay, some pins will be legit, but many are not. If WDW is selling the pin for $8-15, odds are that someone selling it for $1 is up to something sketchy. Think of it this way, if you walk through Chinatown and see Coach bags for $50, or through Times Square and see Oakleys for $10, would you think they are 100% authentic? Pins from eBay are similar. If you see someone selling pins for $.50-$2, quite possibly fakes. $8-10, more than likely real. Look at the sellers history, and other items for sale. Much like our "Coach" purses, if someone has dozens of 25/50 pin lots for sale, or keeps relisting the same item, they are most likely not legit. Now someone could have found that Coach at an estate sale, or an estranged GF, much in the same way someone could be getting rid of their collection and selling them on eBay, but then you would most likely only see 1 of a given item being sold by the person. Most resellers on eBay list the same lot multiple times using the same information.
Pins that come in little individual ziploc baggies? More likely than not, fakes. Pin lots that state "Pins shown are an example only, you may receive different pins", likely fakes.
The seller may include something like "100% tradeable", which may technically be true. The offical rule is
The main criteria to judge whether a pin is tradable or not is that it must be a metal pin bearing a “©Disney” mark on the back that represents a Disney Event, Place or Location, Character or Icon
Are they really going to tell little Johnny that he can't trade because his pin is fake? And even if the CM did care to try to identify the fake, some are so good that even the pin group has problems telling that they are fake. And in a few instances the only way they could tell because they pulled out the pin bible, and the pin they were looking at was never actually produced.
Now if you have the pins in hand, there are a few things you can look for.
Check for clean edges on the pin. Often non official pins won't have all the flashing removed from the edges. This is especially obvious around small delicate areas, more than often they will just leave the excess material and not bother to trim it.
Colors. Non official pins can end up using funky colors. Things like a radio active yellow/green for Donalds beak or Belles dress. Something that we as a WDW guest might know isn't right, but someone unfamiliar with Disney might not know is wrong.
Backstamps. Several things to look for. The most obvious tends to be a backstamp that is too small. Some of the pins have a Mickey head pattern on them, that should run off the edge. Fakes, the pattern will sometimes stop just short of the edge, so that Mickey just gets cut off. Others have the pattern running in the wrong direction (the heads should be north/south facing, on some its east/west facing). Things like that.
Metal/weight. Many fakes are done in a lighter/different/thinner metal. So holding a fake and a real pin you would notice the difference.
Now keep in mind that many of the pins on the CMs lanyards are going to be fakes. There are lot of people who buy the lots and then try to trade them for reals. So even though your pin may be 100% legit, the one you receive back may be fake. As I mentioned before, most CMs don't know/don't care, so just keep that in mind when making your decisions.
Everytime we go to disney we get a lot of disney pins from ebay. they have always been real with the disney emblem on back and with mickey ear backs. we are planning to do the same. We are going to get 50. for our family of 4. but my kids are little so they are just getting into the pin trading. my husband and I love it so we do a lot.
Hate to break it to you, but those are very probably fakes. These are often made in the same factories, so have the back stamp and mickey pin backs doesn't mean anything. And remember, factories in China can make functional fake electronics, so knocking off an embossed stamp or a rubber piece is childs play to them.