That auction, if they have a dealer license, is responsible for supplying paperwork with details of the sale, including the sale price. I'm not sure how they dodged that responsibility. If they do not have a dealer license, then my bet is the boat was sold illegally.
Worst case, contact your local police department and explain the situation to them. BE STRAIGHT WITH THEM! They can do a search, and provide an inspection. From there you can prove the boat is not stolen, and the fact you own it. Getting it registered shouldn't be too big a deal from there.