Hello,
When you build a wall between merchants and their boat (or if the wall aready exists), they die, leaving stuff and bodies. I don’t know if it’s intended but it’s very lucrative.
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Hi Lordthelos and welcome 
Yes, isolating anyone from their go-home route with walls or barricades will eventually kill them - Trader, Pirates, Goblins and your own villagers.
Perhaps eventually this can be fixed with some sort of path restriction or timeout coding, or maybe some sort of penalty for actually doing it, but at present it is just a known problem.
Thanks for your post 
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Welcome!
I guess the easiest solution would be to just teleport the trader back onto the ship.
If I make it so they get angry when they can’t find their way, it could happen that they can’t find their way without it being your fault and they’d get angry anyways.