|Lion of St. Mark, Piazza San Marco, Venice|
From a historian's perspective, the issue of Mark's sources requires constant attention. It's a serious frustration to the historical study of Jesus that the Gospels' evidential base is so obscure. It seems possible to me, however, that Mark's author does give us a hint as to his sources. From the time Jesus called his first four disciples (Mark 1:14-20), the author constantly reminds his readers of the presence of the disciples. They were, as the author tells it, Jesus' constant companions. Is the author of Mark reminding his readers of his "ultimate" source? Can we assume that the author believed his sources were derived from the disciples? That's possible. It at least presents us with the possibility that this gospel rests on fairly solid evidential ground and included sources close to the actual events of Jesus' life and ministry. It's even possible that if a historian had had these same sources she could have written a credible biography of Jesus. That's pure speculation, but it's not entirely out of the realm of possibility either.
My own sense is that the author of Mark had good sources for his purposes, which was to tell the gospel truth about Jesus. We should constantly remind ourselves that gospel truth doesn't necessarily contradict historical truth. It just doesn't pay attention to historical issues. In other words, there lurks in the gospel truth about Jesus data for the historian, data obscured by gospel conventions and concerns but valid data none the less. That data, finally, comes from eye witnesses, though not directly or "purely" so.
2012 Comment: I've referred in these additional comments to Richard Bauckham's book, Jesus and the Eyewitnesses: The Gospels as Eyewitness Testimony. I suspect that Bauckham would reply to my 1998 commentary above by saying that the historical data only seems to be "obscured by gospel conventions and concerns" because we moderns are ignorant of how history was written in the first century. The mention of the disciples is important because in that day it meant that the disciples and other contemporaries of Jesus were the sources for the gospels. That is the way ancient historians and biographers handled citations, footnotes if you will. Bauckham also contends that the historical data used by the gospel writers does indeed come directly from the eyewitnesses, although sometimes through the memories of people the eyewitnesses told their stories to. The point is that according to Bauckham when we read the gospels we are reading an ancient form of historical literature complete with proper citations—we just don't recognize the conventions, that's all.