An item, let's just say a bottle of water could be valued differently by different people. It could be worthless to someone that owns a freshwater lake or worth its weight in gold to someone dying of dehydration.
Trading items with people could be a whole lot easier and the returns could be greater if you have the right kind of item to trade with. However, there were some cases when there was an item being traded that both of you had more or less of equal worth to the item being traded with.
That was when negotiations start. Which I fuckin hate if I do it with the wrong people or for lack of a better term, dumbasses.