I would never trade a pin with a CM that I knew was a scrapper. Even if the CM has to take the trade, that means the pin is being passed off to another person that was probably hoping for a real pin. I have seen some people take scrappers, cut the post off, and turn them into magnets or decorations for hairbows, which I think is a neat idea.
Even the quality control on real pins isn't that great, but scrappers tend to have rough edges or the Mickey waffle pattern on the back can be rough. If you are trading with another guest that has all their pins in tiny individual baggies, most likely those pins are fake also. It's possible they are just trying to keep each pin in good condition, but in my experience, the bulk lots bought off eBay come in tiny clear bags.
On many days, a group of people gather at Epcot near Pin Central and trade with other guests. Many are nice and legitimate traders (I pop in there sometimes)! Some are "pin sharks" and trying to take advantage of new traders. If you are interested in one of their pins and they "suggest" that you buy them a pin of their choice from Pin Central in order to do the trade, this should be a red flag and I believe is not allowed by Disney.