Wait a minute...ding ding ding...is FG trading shrunken Maori heads to naive non-native traders? ("Yes, this head is from a former Maori bigwig, see the tattoo on it, only high ranking Maori have that!")
***** SPOILER *****
Moko, or facial tattoos, are worn by Maori of great mana (prestige, influence), and particularly by tribal chieftains. In a civilisation without a written alphabet, the tattoos showed the whakapapa (genealogy) of the chieftain. So, a glance at the chieftain's face would reveal who he was, who his ancestors were, what his tribal connections were, and, thus, reflect his level of power and his life achievements.
As such, in cutting off the head of a chief whose tribe you have defeated and sticking it on a pike outside your pa (fortification), you are effectively displaying your detailed curriculum vitae to visitors and invaders alike. By the time James Cook arrived in New Zealand, hundreds of chiefly heads adorned pikes the length of the country.
The abovementioned Mr. Cook was presented with a gift of three chief's heads upon his arrival. Marvelling at first at the quality of the workmanship and pondering as to the materials used, it soon dawned on him that these works of art were organic. He nonetheless liked them, and early traders/whalers/sealers/colonists soon began to regard these heads as exotic collector's items. Demand soon began to outstrip supply.
These Europeans occasionally offered money for the heads, but often something more valuable - muskets. Muskets were a must-have accessory if tribal warfare was on your mind, and were the most surefire (sorry) way of obtaining wealth and resources from other tribes.
Frank-Greg, having run out of authentic heads of conquered chieftains, soon hit on an idea to keep the steady stream of muskets rolling in. He would offer his slaves free elaborate (but syntactically gobbledegook) facial tattoos, immediately cut their heads off, and trade them to Europeans with a collector's eye. He would use these newfound muskets to subdue more tribes, collect more slaves, and continue his steady supply of heads.
TL;DR spoiler: This was part of a cunning plan to forge chieftain's heads to sell to unsuspecting European collectors.
Well done, Grip, and some excellent questions from Antesse and Redwine.