Why did captain cook lay out a set of rules for trading with the people of tahiti?

Upon anchoring in Royal Bay, a large number of natives aboard their canoes came to their ship with offerings of Cocoa nuts. These nuts appear to be of great value to them. Captain Cook welcomed the natives on board thinking that he might find some use for them in the future. Knowing that their stay would not be long, he thought it to be necessary that some form of order should be established before they conduct any sort of trade with the natives. He reasoned that the merchandise they have on board should serve a proper value and not be used for everyone's own fancy. Such disregard could possibly cause confusion or fights between them and the natives which would certainly cause their merchandise to decrease in value and be rendered hard to trade off.