Quantity is the name of the game, and you're holding all the cards. When you buy and sell goods, you get a better deal for larger quantities of a single item.
E.g.: John has Bulk Trader. When traveling, he notices that all of the Blackspear tribals carry spears along with their assorted equipment. He goes out of his way to save up a few dozen of them for a rainy day. When he has about fifty, he and his companions haul the load down to the weapons dealer for a bag of cash.