Can one give or sell non-Kemach Yashan products to a Jew? Is this like causing them to sin?
You can’t sell something that isn’t kosher even to a non-Jew, since we are afraid he might sell or give it to a Jew. But chadash has some poskim that permit it. Therefore, if the Jewish customer is ot stringent and eats these foods, it is fine to sell it to him.