All pins should have the Official Pin Trading or a Disney(c) mark on the back to be eligible for trading with CM's.
These are just the guidelines for what pins are eligible to be traded in park. Basically trying to trade an Olympics pin for a Disney pin isn't going to happen. (Unless that Olympics pin was somehow cross licensed by Disney)
Buy on ebay to start out. Make sure you buy from an experienced seller and read their feedbacks.
Read the feedback, but more importantly, look at what they have for sale/have sold. If a seller constantly sells lots of 25, 50, 100+ pins, and almost always uses the same description, "Guaranteed tradeable, no duplicates, selection may vary from picture", those are pretty much going to be fakes. If the lots are shown by picture, text isn't copy/pasted, and the user has variety, including single pins, sets, etc, then they may be real.
With the current state of trading with the CMs, they typically don't care, or don't know how to spot a fake. If you are looking to trade just for the enjoyment of it, and don't care about the value, then you know what to do. Trading with knowledgeable guests, or hoping to get pins that will have a higher value, heck, even some value, most pins from the bulk lots won't do you any good. Many serious pin traders will actually do what they can to remove scrappers from the market. Saw a posting from one who took a bunch of fakes they had and mixed them in with the concrete when they were doing some construction on their house. However, there are more than enough pins that have been faked that are easily available on CM lanyards to still make a good collection of it. For instance, of my 30-40 vinylmation pins, I think that maybe 10-15 are legit. Some of the others are obvious fakes, and some are questionable. But I'm not looking for value or long term worth, I'm looking for pins I like.