Although a Sunfish is a Sunfish by design the Sunfish has had five different manufacturers. From the start, being produced by 2 men, until now being produced by Vangard others have produced the Sunfish (maybe AMF, Pearson, SLI?). The boats of one maker seem to suffer more problems than others. You may want to check the Class website under FAQ and click on "How to tell the age of a Sunfish". It will give you clues as to the age and builder. It also notes what boats to avoid unless you really know boats. I am sure I have seen a more detailed explanation of what to look for when buying a Sunfish on other sites, but I do not have the link to them.
Always lift a boat before you buy it. If you hear water slosh, DON'T buy it unless you want a project. Turn it upside down and look at the hull. If you find holes, cracks through (not minor crazing), DON'T BUY IT. If two adults have to struggle to turn it over, DON'T BUY IT (should weigh 130-140 pounds, takes some effort to turn over, but not a major struggle). If there's water inside, LEAVE IT, so the next prospect knows what they're getting into!