Being able to meet the parents of the puppy also helps a lot with knowing what to expert for temperament. Gentle, social, calm, stable doggie parents are a big help in producing gentle, social, calm puppies. If the parents are... uh... unstable (i.e. "will attack strangers, so no you can't meet them"), I'd run away from that deal, too. Mastiffs may not like strangers, but they shouldn't attack... if the momma dog is still protective of the puppies, they should still be able to let you see her interact with her own people (from the other side of a window or fence), so you can get an idea of her personality.
Not sure about the middleman, either... other than it can take a lot of time to find a good breeder and a great puppy, so his network of breeders may have some value there. Maybe offer him a 'finders fee', but tell him you need to talk to the breeder direct and be able to see where they spent their first 8 weeks of life.