A while back I went through and analyzed all the marketplace startups that YC had invested in, and from a mixture of gut experience and the data it seems like YC is less likely to accept marketplace startups than non-marketplace startups (i.e. first time founders disproportionately found marketplace startups but this isn't shown in the YC cohorts). The ones that YC have accepted have generally seemed to been where they've really liked the team (i.e. AirBnB) as opposed to the idea.

This is actually somewhat different to most funds who invest in marketplace startups, generally for most investors there's only one thing that makes a marketplace startup worth investing in and that's traction (because in most cases the technology itself is trivial).