Could be blowby on an Exhaust manifold gasket... or oil leak from head gasket leak...
You pretty sure the odometer is correct ?
How much are they asking for the car ?
Does it run well ?

Your best bet is to spend about $100 and have the car really checked out by an independant company.... not your potential mechanic... someone like lemonbusters... who do no repairs.... only test the cars... so no conflicts of interest... either overt...or subconscious...
They will do a compression test and lots of other tests......If they had gotten in there and cleaned that spot off then engine you would not even be thinking about it.... so mechanical tests are way more important than how it looks in the engine compartment...
Someone can keep the engine spotless.. but not use the correct oil... or change it often enough... and you get a false sense of value...