One is not able to sell one's soul to satan for the simple reason that one is not the legal owner of your own soul. Your soul has been purchased by Jesus Christ on the cross. You can do a lot of sketchy shit, but the price has been paid, and ownership has been transferred. The idea that it can be sold by a non-owner is as false as thinking you can sell your neighbors Buick. Like everything else satan says, it is a lie.