Final answer:
The Elven king takes them as prisoners because of a law that requires white men to inform noblemen and officials before purchasing goods.
Step-by-step explanation:
The Elven king takes them as prisoners because of a law that requires white men living in their kingdoms to inform three noblemen and officials of their court before purchasing goods. These noblemen and officials would then investigate if the goods are captives or free men. If the white men do not comply with this law, they would lose the goods.