Is this your only concern in purchasing this boat, or is it one of many? Have you had a surveyor look at it? If you see no evidence of water penetration or structural damage on the underside then perhaps it's just an inevitable product of stress and strain given the age of the boat. If this boat has been trailered on a regular basis that means the mast has been raised and lowered many, many times putting added stress on the base. Have the seller/ owner drop the mast and then check the base for movement.