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You would have grounds to sue the seller for breach of contract since your payment of deposit gave you the rights to the boat. Under a breach of contract suit, you have the right to recover all damages directly associated with their breach of contact, but only actual damages can be recovered and no emotional or inconvenience damages.
You can only sue for what you prove your actual losses are, which could be any increase in price over what he was charging you if you find another boat of the exact same make and model.
You would have to sue him in MO courts, as those courts are the only ones with personal jurisdiction over him.
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