Well......if I ask a CM for a pin and it is a scrapper I give it back. I just say I changed my mind, they have never questioned me for doing so.
Also, I just accept the fact that I'm likely to get scrappers when trading, I try not to stress over it and I try to repurpose the pins into a different use. IE breaking off the post and making them into magnets, using them as push pins, using them as bathroom decor. There really is no way to make sure you don't get scrappers, unless you just buy pins and never trade.