Satan as we know him today isn't a Christian or even a Jewish creation. The Jews had Lucifer as the accuser long before Satan as we know him existed. The true Satanic figure comes from the teachings of Zoroaster which posited two gods in duality- a God of Good (Ahura Mazda) and a God of Evil (Ahriman). This was, when you look at it historically, a blatant political tool that was used against the populace. Whether it was intended that way in the beginning is debatable, but it didn't take long for the politicians of the time to see the use of religious fear tactics.
The Jews picked up the idea of the Satanic figure during the Babylonian exile, and it was fairly popular among Jews during the times of Jesus and Paul. Since they both seem to have had a strong belief in Satan, or at least in the stranglehold he gives on people's minds, the Christian faith got a good, strong dose of nastiness from the very beginning. He wasn't something created by the Church- He was created long before the Church was ever conceived.
About the question of 'is he real'.
Well, it depends on the beliefs, few people worship him and few do not even believe in him
You can choose your own path and if to believe in it or not.
Selling the soul is purely propaganda. Made to make it sound evil and vile. It was primary used against witches since they claim they sold their soul for power. There is no Soul selling or even sacrifices in Satanism.