It's not like you just FIND a rare monster that doesn't want to be found! You have to go out looking for it. I also had a drawing of the heaps of treasure. I actually did a bit of actually research while doing this, to find out what sort of things people would have traded in. Pirate in particular favor silver coins over anything else as they were easy to distribute among the many. Rope, tar, and textiles were also pretty common, as well as food stuffs. On a sea voyage food tends to spoil, so fresh supplies are very valuable, especially fruits which staved off scurvy.