That's a tough one. I would do lots of research before it gets shipped. Problem with the shipping companies, they are really vague about what they will insure when it comes to high dollar items. I would surely use the premium service if available, such as FedEx overnight, or UPS overnight. It's going to cost a bunch, but if you lose that watch, someone is going to be out a pile of money!! Before shipping make sure you and the seller are on the same page with everything. Have him take several photos of the watch, the watch in the packing box, and then after the box is sealed and ready to ship. Photos with the address label, and customs declaration form, to establish that is the same box.
I would declare it for it's full value and have it insured for that amount, as in most cases, you can only buy insurance up to the amount of the declared value. The seller needs to treat this as a genuine watch, not a 200 USD rep. He really should find out which of the shipping companies will ship high value items with full insurance. Some will tell you, "Sure we will insure it for up to 25K, but if it gets lost, they show you that minute print clause that says watches and jewelry are only insurable up to 500 USD. I know that watch and jewelry dealers have special companies that write insurance for high value items. Things get shipped all the time, every watch, diamond ring, bracelet, etc. has to be shipped, probably multiple times, from the manufacturer to the selling dealer. Someone is insuring that stuff every step of the way, you just need to find out who and if it's available to individuals.