Tell the buyer that you want a non-refundable deposit (preferably cash) of 10% as proof of his/her sincerity and stop accepting other bids.
Agree upon a closing date in which the balance of of the selling price will be transferred into your account. The sooner the better. I would give the buyer no more than 30 days to complete the money transfer.
DO NOT allow the buyer to move aboard or tinker with the vessel in any way until the transaction is complete.
After you have verification of the money transfer from YOUR banker, accompany the buyer to the local registration authority and both parties sign on the dotted lines.
Here on St Thomas there are no taxes rendered... other than the local registration fee, which should be paid by the new owner. I sold my last boat here and have assisted others with the process. Keep copies of all documents.
Shake hands. Go sailing one last time (if invited) and show the new owner the little tricks you've learned about your old boat.
Check into a nice resort for a few days, relax by the pool and reflect on the good times your old boat provided and contemplate your next big adventure.