A 35 year old $700 boat- there could be any number of things to look out for. At that price, unless the guy is really just looking to unload it quickly, it is probably a lot of original equipment so it is likely getting "tired". The good news is that 1980 hulls should be well built, but I would still inspect all the standard stuff - soft spots, cracks under the hull lip at the crossbar connections, cracks in the transom from the rudders not kicking up properly ( a 1980 boat would have had the old style rudder system).
Inspect the condition of the sails. If they are original, they probably don't have much life left. Inspect the crossbars and other aluminum parts for cracks and corrosion, especially if the boat was sailed in salt water. Be sure to check the front crossbar near the mast step as this area is known to crack. I believe a 1980 boat would also have the old style 2 rivet mast step.
Inspect the wire standing rigging and if the owner can't tell you its age, be prepared to shell out a couple hundred bucks to replace it. An of course make sure the boat is complete and all parts are in working order.