When you think about it, salt is very, very easy to produce and harvest. The old Hawaiians took water from the ocean and made salt beds from which they harvest the salt. Not only is this a common practice in Polynesia but also in Asia where Indians and southeast asians produced salt from the ocean. But in dark/Middle Ages europe, salt could only be obtained through trade with Africa and Asia. Salt production could have easily been produced in Europe via the Mediterranean Sea and Atlantic it's not a rare commodity if you think about it.