How to get to Secret Vendor in Salty Mast - Pillars of Eternity Guides
In order to get to the vendor on the second floor of the Salty Mast you'll need to speak with the Godlike by the stairs on the first floor. He doesn't have a unique name, he's just Older Drunkard (pictured above). He'll give you the ring that you need to gain entry to the room on the second floor. I recommend putting this in a place in your inventory where you'll remember not to vendor it.
The vendor on the second floor of the Salty Mast along with the vendor in the Doemenel are two of the only vendors in the game with a wide range of rare items for sale. Sadly this vendor will never restock so once you buy all of the unique items from them there will be no point in coming back.