It has been my experience when searching for Haldor the vendor, all one has to do is sail close enough to the island he is located on and a bag icon will appear on the map disclosing his location. When one gets close to an island while searching for Haldor, open the large map regularly until the bag icon appears. Sailing around an island he is located on will make the bag icon appear on the map, no need to land and search an island on foot. Sometimes the bag icon does not appear on the mini map, until the fullscreen map is opened when one is in proximity to the island he is located on.