Look for oil leaks around the front cover, oil pan, front/rear seals, look for signs of oil leaking, look for signs of rusting or corrosion. If stored outside look to see that there isn't signs of water seeping inside. I'm assuming this engine is already out of the car...chances are you can still get the compression test done if you care to. You just pull the plugs and connect a battery with jumper cables to the starter to tap until you get the thing to turn over and give you readings. Oh and when you pull the plugs be sure to inspect them to gauge how bad (or good) they look like.
A seller will tell you anything just to get rid of it...so just be careful and cover your behind! Plus what does 'ran strong' really mean? I want to hear it ran great, it ran smooth! :P