You have no way of knowing if it has been serviced unless there is a receipt from a watchmaker. If this guy says they are serviced then you would have to take him at his word - it doesn't say that in the adverts though. As a minimum indication, if the case back is dirty, then I would not believe the watch is serviced. A movement picture is also nice.
Have a look through all the watches mentioned here and see if you think all the case backs are clean. As I say, that's just a minimum indication.
People everywhere seem to be asking a lot of money for these watches but, the only way to get any sort of a deal is to spend time, I'm afraid. In a few days, you just don't have the time. There's a lot to learn here, as examples of what is usually true: Automatic watches are more expensive than manual wind; steel watches are more expensive than gold plate or gold cap. Lots of rules!
Regardless of the service, some of the watches mentioned have marks on the dial (usually scratches) and I can't see you being happy with that for years to come.
Good luck, Chris