Selling a soul is possible and is not at the same time , the only actual relevant fact is if the person you are listening to believes in it or not. So i wouldnt listen to 50% of people here for example a heathen who does not believe in satan will tell you that he does not exist while a christian or a person open to all forms of knowledge like myself will tell you its possible.
Selling your soul is a highly foolish idea , your soul would be imprisoned after death , there are a few rare ways to get out of the deal which i rather wouldnt discuss here. The "gifts" that you get in return are very bad when you look at the big picture. This is a small ammount of knowledge that my teacher has gathered from all practices and religions , people who deem their opinion as true knowledge shouldnt teach , unlike my teacher i respect the right for people to have their own opinions on things but thinking that its the onlybtrue knowledge is pure ignorance. So be careful whom you trust princess pain.