Why did Captain Cook lay out a set of rules for trading with the people of Tahiti?

I would say he wanted to have good relations with the people of Tahiti by having clear rules whose purpose was to prevent the Tahitians from thinking they were being taken advantage of and in which their trade goods would be exchanged for things of at least equal value to avoid conflict. This would allow them to peacefully reach their goals one of which was to make observations of the planet Venus.