The key phrase is that non-citizens have to declare the value of any goods they are bringing into the United States that will remain in the United States.
if you are entering the US with a watch that is your own and that you will leave with, you don’t need to declare it.
if you are entering the US with a watch you intend to sell, then you should declare it and pay customs.
of course, this is what US law says you should do. What you choose to do is up to you.
one other thing to be aware of is that Rolex USA has vigorously sued on trademark grounds over the years enough that US customs interprets the code to mean that if you are bringing more than one Rolex into the US, the second is automatically considered merchandise for sell AND you have to have permission from the trademark owner (Rolex USA) to do so. If you don’t have Sid permission, the watch can be seized and turned over to Rolex.
here’s a copy of that actual ruling:
https://www.customsmobile.com/rulings/docview?doc_id=HQ+H071620
so, if you are going to bring it over, I would not have any other Rolex in my possession, just to be safe.