The name may make it seem as if the sailors made the boxes themselves and originally it was believed that they had, but the items needed to do so would not be available to them, such as hinged boxes and shaped glass covers.
Actually it was the business venture of two Englishmen who brought about the industry of the valentine.
The truth behind the origin of the original antique boxes became clear when they needed repair. It was discovered (through papers found inside) that most had been purchased at shops on the island of Barbados.
Entrepreneurial brothers from England, B.H. and George Belgrave, had set up a "Curiosity Shop", (or two as this article in the Barbados Advocate says), in Barbados that sold many of the valentines.
The Belgraves were wise to realize this business opportunity by mixing seashells, which were covering the area I imagine, with the many sailors who became their customers at the popular port. It was a lucky combination of supply and demand.
(Seashell photo by P. Carter)