Refers to a secondhand automobile. More formal or technical than 中古車 (ちゅうこしゃ), which is the common everyday term.
He bought a used car and repaired it himself.
This shop sells and purchases used cars.
中古車 is the standard, everyday word for a used car. 古自動車 is less common and can sound more formal or technical.
Compound of 古 (ふる, 'old') + 自動車 (じどうしゃ, 'automobile').