If you have a Piece of Eight in your inventory, you can ask the Innkeeper about it:
Piece of Eight
Look at that! You've got a piece of eight. That's no common coin you've found. It is a coin that was once used by an ancient civilization that lived on these islands. These days, they are still used as a form of currency among the most prestigious, including high ranking pirates.
You see, the ancients used these coins similar to how we use gold coins now. However, over the eons, most of them have been lost. Mostly consumed by the sea, I would wager. Now days, it is quite rare to see one.
It just so happens that I am in the market for some of these pieces. I have been trying to barter with a fellow for an artifact that I wish to add to my personal collection. However, he is such a self-proclaimed high brow that he will not even raise one of his brows to the offer of a pile of gold. He says he only deals in pieces of eight.
He requires seven pieces of eight for the trade. If you can obtain seven of them, I have a trade to offer you. I recently commandeered a beautiful ship from one of my enemies. I sent him paddling off in a small log raft after I took his ship. Cruel perhaps, but he knew better than to cross me in the first place.
Bring me seven pieces of eight, and I'll trade you my new ship.
The Piece of Eights can be found while looting the bodies of the various pirates you encounter. They will be found on bodies at random, but remains very rare! So one will expect to kill many pirates in order to obtain all the pieces. Once you've got at least seven of them, return to the Innkeeper and he will say:
Deliver pieces of eights
You've found the pieces. Impressive! As promised, you can have my new ship. Now I can finally trade the scallywag for that beautiful excessively feathery ha... I mean the rare artifact for my collection.