In Satanism there is no such a thing as "selling" a soul to Satan. Deals maybe. Although do not look at those deals with an eye that Satan will give you something and in return you give your soul. Deals actually are prayers. You pray for something to Satan and he might give you it if you are worthy for it. Keep in mind..you actually make a deal with your faith and love to Satan. That is what Satan wants.
There might have been cases of deals with Satan that are completed throughout history. Many deals included Popes of those times & Kings/high in command people.
Everyone wants his own success in life. If "God" does not give him/her what he/she pleases then people will go to our lord Satan. Satan will help them, he helps everyone but in the afterlife Satan will punish those that used him to cause harm to others.
Pray to Satan, he will help you. Do not use him to harm but rather use him to improve yourself and the society. Spread his word and do not believe the nonsense that you "sell" your soul.