As I stated earlier, I'm not an expert - so I can't comment on specifics about the watch. But I found your other thread and found the listing for the watch.
The site where it's listed for sale is legit. Does the site post any reviews or feedback of sellers? Does the site do any authenticity checks or are they just simply a site for selling? I have not purchased through that site, so those are questions I don't have the answers to.
The seller is basically non-existent outside of their sales postings on that site. They are not on any other typical sites to check feedback and/or reputation. Are they active on any other sites - watch related or otherwise - with feedback in any way, shape, or form? If so, can they prove that they are that person?
Have you contacted the seller directly asking for specific pictures of the watch next to a current newspaper and set to a specific time? Have you run the pictures they posted in their sales listing through Google's reverse image search? Have you asked to call and speak with the seller?
Are you local to the seller? Would they meet you for a face-to-face deal - perhaps at an AD?
Do they allow you to get the watch inspected at an AD and return it if it's not legit?
Is a 30% discount typical for IWCs?
Will the seller accept PayPal? Do you have a credit card with buyer protection to pay through PayPal? If you end up losing the money, can you live without it?
After communicating with the seller, do you have a good feeling about them? Or are you still not comfortable with them?
These are all questions that would go through my mind and guide me to my decision if I were in your shoes.
Hope this helps.