Actually the import duty is 1.5% (see the link above). You can not attempt to sell a foreign flagged boat if it's in the US unless you state this:
"Why are some yachts “not for sale to US residents while in US waters
You will see this term next to many yachts advertised for sale
. Any non-USA built vessel which has never paid USA Customs
duty, or a USA built vessel that has been exported and is currently foreign flagged, can not be offered for sale
to US citizens while in USA waters until making a formal USA entry. USA duty, assessed at 1.5% of the appraised value, is a one time taxable event. A duty free formal entry can be made if duty had previously been paid and can be proven with a receipt or a returning USA built vessel."
Frequently Asked Questions | superyachts.com
The buyer might have to state register the boat which can be from 0% to 6% depending on the state. He will pay this.