From personal experience, there are
CM's who can tell the difference between a real pin and a scrapper, but they are few and far between. They are allowed to swap them out for real pins if they do come across a scrapper, but trading a scrapper to a CM is still extremely frowned upon.
A lot of cast members at least know when they are being traded crappy pins, and trust me, they do not like you for it. Especially when people start pulling the pins out of tiny, individual plastic bags,
most CM's are trained to know that it's a scrapper. I would usually inform guests, politely, that trading fake pins is against Disney rules, because it really is. It also sucks as a CM having crappy pins on your lanyard just because some kid's mom found a great deal on Ebay, because the next kid that comes up to trade only sees the same pins they have seen 40 times that day on other lanyards because the same scrappers are everywhere, because you all bought the same crappy pins. Just spend the extra $5-$10 on real pins from reputable people on the FB pages already mentioned and make the lives of everyone easier.