Just tucked away at the very end of their rather patronising "beginners' guide to collecting Classical Marbles", Christie's have this to say:
The collection history of a piece is very important to consider. [...] A savvy collector will always ask for all available information, and responsible dealers and auction houses will endeavour to provide as much as they can.[...] If you feel that there are some unanswered questions, or you don’t feel 100 percent comfortable with the information you’ve been given, walk away.Thus absolving the dealer of any responsibility, they've "done their best" and if you don't like what you see, buzz off.
They suggest: "With online archives becoming bigger and better, you can do a lot of research yourself on collection names, locations and specific objects", Yeah? But you cannot do any research on items that surfaced ("from underground") until after c. 1995 - which is about as far back as any online auctions will go. But of course retrospectively compiling (making up) a collecting history for an object with a discarded one is no substitute whatsoever for buying from a responsible dealer who only fills his or her stockroom with objects of firmly documented and not merely reconstructed collecting history. Note Christie's studiously telling the beginner WHY that collecting history is important - avoiding that "i-word". It's nice to know who famous had it before you is what they say. The warm fuzzy feeling you get from that has a "real added value which will not diminish" they suggest.