How is it that after we do Bedikah (checking for Chametz) and we are Mevatel (Making the Chometz Null and Void) the Chometz the Rav then goes and sells the Chometz to the Gentile we have alredy said it is like dust of the earth how are we selling something that we said no so long back it is like dust of the Earth Either the Sale is false or the Nullification is?
After the Bedika, the nullification formula we use is only for chametz that we don't know about. This leaves out the chametz we've set aside to sell or destroy. We don't do the nullification of everything whether seen or not until after we burn chametz the next morning. By that point, the sold items have been sold and are not subject to our nullification.