If the boat was built in the 60's then I bet that it is rivited not welded.

Break down the componets, Boat $1000, Motor $1000, trailer $500. Lot's of factors to consider with older boats, but to me it sounds like $2500.

The market will tell you the value to a point. If you ask $3500 and no one even makes an offer or calls, lower the price.
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Many of my best trips have resulted in a short limit!