If you are accepting a bill-of-sale on US Coast Guard Form CG-1340 (downloadable on the USCG website, do not accept any other form of bill-of-sale) and the seller will be signing in a foreign country (St. Kitts) you must have it notarized by a US consular official and/or an apostille certificate must accompany the CG-1340. Note: The SELLER signs the bill-of-sale, not the buyer.
As for the airline issue,
buying fully refundable round-trip tickets (known as a full Y class fare ticket) is an option but they are much more expensive than
internet offered cheapo one-way tickets. I’ve never had a problem with these. I carry a well-drafted, official looking “Crew Letter” that states that I am the master,
deck hand, cook, etc. assigned to sailing vessel X (include vessel name and official document number) located in country Y. I will be departing country Y on said sailing vessel on approximately date Z. Include your USCG
license number or state boater’s operator certificate if you have one and your passport number on the Crew Letter.
There was a good comment on this string about calling the airline in advance to let them know you are on a one-way ticket on a Crew Letter. They will post this info on your PNR, airline reservations talk for “Passenger Number Record” stored in their computer system. Any counter or gate agent can pull this info up if they give you a hard time. In fact ask them to put this on your “PNR” and they will think you know you stuff.
As far as clearing out of St. Kitts, every country has its own procedure. Local knowledge is essential in these cases. The more documents you have to prove
ownership of the boat and yourself the better.