First and foremost the best way to buy a used recent/modern watch is from the original owner. Most of the time they will have cherished their watch as they never dreamed they would selling it when they bought it. You are likely to get a watch that has not been "touched up" for resale by a buff here and there which is a big deal with PP as they won't polish watches buffed by anyone else. Plus you can get all the original papers and boxes. If you are near the seller by all means go look at the watch. I get concerned flying on planes anywhere to see a watch with a stranger, maybe that's just me.
I would confirm they have the watch by having the seller email you pictures of the watch set at certain times of your choosing along with a copy of the COO. They may delete serial # which is common. You need photos of the watch which fully show the condition.
I would get some reverences. The AD should speak highly of them. I always give my banker and as many names of ADs as they need. Its great if there is feedback on the net.
I get a lot from talking with people. You can get a sense of their passion for watches and what they are now looking to do. Its got to feel right to do the deal. If the price is really low, the deal may not feel good. But modern used watches save a few popular references, sell at 50 percent of MSRP or lower. Its all about demand and scarcity.The prices you see advertised for used watches usually do not reflect what an individual seller of a used watch could hope to obtain.
The best way from the sellers standpoint is to have the money wired. Its seem to be taking less than an hour now. Not like the old days where it could take all day or into the next day.
If your good to go, I would wire the money. I would not personally travel by air to complete the transaction. I think you needlessly incur risk. A car I guess makes sense as its convenient and cheaper to drive it back home.
Just my two sense.
Enjoy your new watch!