Once upon time, there was a shallow inland sea, leftover meltwater from a previous ice age trapped because of nearby mountain ranges. The arid climate with almost no rain and weak flow to and from the sea ensured that the water became salty over time. Especially common was the formation of brine pools at the bottom of this sea.
Then the sea slowly began to recede as the ground was rising, leaving only a thin strip of brine pools and later salt plains after the last water had evaporated as well as unpleasant marshes and plenty of salty little lakes.
Or so I have been told about the history of that inland sea. Anyway, there is a problem with those salt plains and their salt: something makes it useless for trade or unwanted for everyday usage. I have confirmed that it must be something in its composition or contamination that cannot be removed with late iron-age or very early medieval techniques. Otherwise, people would be flocking in and harvesting this salt as the marshes are no more dangerous than marshes of our world—maybe even less without those pesky salt water alligators around.
It cannot be an overly toxic reason, because one can (barely) live on those marshes, and some tougher flora and fauna exist. However, one needs be very desperate to live in those marshes. Or, unlucky enough that their story takes them into that place.
So in short, what is in this salt of those salt plains which makes it useless?