Translated word-for-word from Spanish to English, "partes usadas de carros" is "parts used of cars," which means "used car parts" in English. Spanish constructions and grammar rules are different than those in English, so some word rearrangement is necessary to obtain the phrasing for each language.
In Spanish, adjectives most often follow the noun and agree with it in gender and number, but this is not the case in English. "Partes usadas," combines the words for "parts" and "used" in Spanish, with "usado" modified to match the plural feminine "partes." As ownership or origin is expressed using "de" plus a noun afterward, the noun plus adjective combination, "de carros," signifies the parts belong to the cars.