I think I am one of your conversation partners on this, and you have misunderstood (in part).
I actually do think Mark probably used oral sources, and some of these may have been eyewitness. And I wouldn't rule out of hand the idea of written sources -- though I kind of doubt it.
My concern is that there is very little basis, if any, for making any assertion about (a) whether any sources were actually used, or especially (b) if they were written. We have no clear criteria to deconstruct Mark's narrative. It works as it stands. And so it might well have been the "creative" work of Mark who wove together oral materials, his own organization, and a rhetorical aim that imagined an audience response, into a unified satisfying narrative.
Mark A. Matson
> Chuck Jones wrote:
> I may have misunderstood my conversation partners, but the concensus
> seemed to be that Mark free-composed the gospel, and that it is not
> correct to think of him has having made editorial decisions towards
> sources, written or oral.
> My point is simply that Mk is written in the same style and structure
> as Mt and Lk, which suggests strongly that all three were based on
> written sources.