Synoptic sources

An oft-made claim is that Matthew and Luke got much of their information from Mark. And it's certainly possible that Luke got some of his information from Mark. 

But I'd simply point out that the inference is fallacious. The fact that Matthew and Luke copy (and edit) Mark doesn't imply that Mark was their source of information.

A historian may copy a source, not become that's where he got his information, but because that's a respected source. 

Likewise, if Mark already covered many key events in the life and ministry of Christ, If Matthew and Luke agree with his reportage, then it's convenient to pick up where he left off rather than starting from scratch. If, in the nature of the case, they'd be recounting many of the same events, why not incorporate this preexisting material into their own biographies, which they proceed to supplement with additional, distinctive material?

If Mark was well-received by the NT church, why not build on that foundation? This doesn't imply that they got their information from Mark.

For instance, a Civil War historian may have multiple sources of information for the same event, yet he may only quote from one primary source to make his point. He might quote an eyewitness like Lee, Sherman, or Grant, because that's a credible source. That doesn't mean the Civil War historian is dependent on that particular source–as if that's his only source of information concerning that particular incident.