Have the seller start the car while you stand at the rear of the car. Watch for any smoke during start up.

Before starting the engine check the coolant for oil & check the under side of the oil filler cap for foam/froth. Either could indicate a blown head gasket.

Also, while you are looking under the hood (before you start it) note if the engine is cold or already warmed up. If already warm, seller may have run it a while to mask any cold start problems.

Just my 2 cents.