I'll second SitePoint. They have some very high profile sites sold on a daily basis in their marketplace.
As someone who visits infrequently I've only seen 10x monthlies which seems ridiculous for a high profile site.
I would define a high profile site as any with a long existence, tons of traffice - but still off the radar, & consistent revenue... or some super flash in the pan breakthrough site like twitter (new & no rev).