What is the watch, does it have service paperwork (particularly from Rolex) and how much is it up for? Buying one without paperwork isn't a problem, especially so if the vendor will guarantee it, but it all depends on what it's up for.
Can't help with the valuation on that I'm afraid. Do you know if the dial is an original Rolex one or a customised one? There tends to be a reasonable difference in value and Rolex won't touch a customised watch.
Probably worth dropping Dom H @ Hackett Watches an email.