It wouldn't be impossible for many people, including posters here who are adament that the go through all the checks, to inadvertently have a stolen gun in their collection EVEN IF THEY GO THROUGH A DEALER OR GET SOMEONES NAME/ID, ETC.. How is this possible? Well I personally had a revolver stolen from me years ago. That was back before I recorded serial numbers religiously. Guess what. I reported a revolver stolen. Could be in your collection of used revolvers!
So a point I'm trying to make is that you really CAN'T always know for sure. And the same is true with the sale of other items on craigslist, yard sales, ebay, etc.
So - why are guns any different? Is it because you can kill someone with a gun? I've seen expensive baseball bats (aluminum ones) stolen from sporting goods stores and sold on the side. They make fine weapons. No serial numbers there.