In a perfect world, your questions will be met with sincere, knowledgeable
answers. However, in many situations the watch offered for sale was not the
property of the seller during the watch's entire existence. For this reason, I
consider the purchase of used watches bearing in mind it will need service.
The best I can usually hope for is, first of all, that the seller is
trustworthy (from feedback, experience, reputation in the community, etc.).
Then I examine photos as critically as my computer screen will allow. Look for
clues like damage to the case, especially the back of the case. Clearly avoid
watches which appear as if some amateur attempted to unscrew the back of the
case with a pair of scissors, or a hammer and a punch!
If the watch appears in fine condition, and the seller is reputable, I will
assume the movement is original, intact, and approximately the same condition
as the case. It can be a risky assumption, but as I say, I only purchase the
watch with the understanding the movement will need service. If the price is
agreeable considering the need for service, I will likely buy the watch. If
the price would be more agreeable with a discount to allow for part or all of
the cost of a service, even better. But sellers often price their watch "as
is" with the burden of service (if needed) left to the buyer.
As it relates to the particular watch you are considering for purchase, I
would budget for service just based on the age of the watch. Fortunately, the
ETA Valjoux 7750 movement inside this watch has a sterling reputation for
durability, and parts (if needed) are available and typically less expensive
than most "in-house" movements. In the U.S.A., IWC's price list for service on
the current version of the watch (featuring the same ETA Valjoux 7750-base
movement) is $655 US, which seems very reasonable to me. Independent
watchmakers will likely charge about the same.
I think the price the seller is asking seems fair for the watch (and
bracelet!) if all appears in fine condition. Perhaps you could ask the seller
if he could adjust the price down to accommodate a service for your own peace
of mind. The seller may or may not agree, but you should have decided at this
point in the negotiation whether or not you are willing to purchase the watch
at the asking price.
Good luck, and please let us know how it goes (and if you get it, share some
photos!).