Well, the man at the place where I get my supplies said it probably wouldn't be a good idea. "Not really" was his answer. Any idea why he may have said that? What are the magickal properties of sea salt?
Salt of any kind represents the Element of Earth, and of course the pentacle or paten can be used on the altar to represent Earth as well. You can also use a bowl of salt or a bowl of earth for the same purpose. I don't see why making a pentacle of salt wouldn't work, but if you're putting it directly on the altar it will over time damage the altar surface, and it would likely lose it's shape while you are doing your working.