I went to the dragon with a large caravan of s2000s and when we stopped to check our oil after driving from Chicago to North Carolina most people were 2 quarts low. My rule of thumb has always been to check my oil at every fill up which happens to be every few hundred miles.

 

Looks like it's simply dirty. Maybe some spilled oil from an oil change.

Either way, it would still be best to have the car taken to a mechanic who knows S2000s for a checkup before buying.

If you could take the valve cover off to check it out thoroughly, I'd check the retainers. There are threads that contain pictures of retainers and keepers that are in good condition, failing, and failed.

Definitely agree with a compression test as well.

 

If his mechanic checked for valve lash I could see them cleaning that area around the gasket seal when they removed the cover.

 

seems like the valve cover gasket is leaking.

I am not a car mechanic so I don't know for sure.