I agree with everyone here, that the #1 priority is trusting the seller!! I think we can all agree that purchasing a watch with box/booklets/papers is worth more than purchasing a watch without these items. Granted the amount that someone will pay for these documents is rather subjective. What really concerns me is the how the watch can become significantly more valuable through its chain of ownership. When the watch is sold from the first owner to second owner, lets say the watch only comes with the guarantee certificate (so no box, booklets, etc). if the second owner is a real collector, he/she may source a period correct box, booklets.
Now assuming the watch is sold from the second owner to a third owner and then eventually to a fourth; not uncommon for a 40+ year old watch. Through ownership changes, the watch's history may change (whether due to ignorance or "exaggerated truth") to the point that the watch now comes with ALL original box/papers!!! Through all these chains of ownership, who really is to say that the box, booklets, and papers that NOW come with the watch were not the original? Granted a $5000 watch without B/P won't turn into a $20,000 WITH B/P, but it nonetheless makes you think about box/papers; specifically their authenticity and worth to a buyer. This of course is not taking into consideration what happens when an auction house gets this watch and its "original" package to auction, and a spirited collector pays a super premium for all the documents and watch.