This is an agreement in International Trade between the original Buyer or Intermediate Buyer and the new Buyer, detailing the terms & conditions of resale. This transaction happens while cargo is still in the International waters, on its way from one country to another. Hence it is called High Sea Sale.

Usually bulk commodities are traded this way. The original buyer will endorse the documents in favour of the new buyer & new Buyer files the bill of entry with HSS agreement & endorsed documents. The Advantage for the original & Intermediate buyer is that their money is not blocked in duty payments etc. and they can exit the deal with their margin.