Sources behind Mark's Gospel - Primary.
Peter : Who was one of Hesus' closest disciples. He betrayed Jesus.
12 Disciples: Mark would know the other disciples as the all met in his mother's house, he would have heard their stories of their time with Jesus.
Other Witnesses: Mark would have met others, who have had their lives changed by Jesus. He may have met Simon of Cyrene, where he learns how Jesus was crucified.
Personal Experience: Mark may have followed Jesus when he was being led by Judas. He may have also witnessed the arrest.
Divine Inspiration: Mark was inspired by God.
Collections of Teachings: Certain people were chosen as called by God to be teachers and preachers.
Passion Narrative: Events leading up to Jesus' Crucifixion wold have been told during Eucharist.
UR Markus: Mark worked from an earlier gospel writen in Aramaic.
Paul and Barnabas: Did not know Jesus personally, were taught about his life and teachings.