Sometimes it's easier to buy already a built car, rather than building a new car from scratch. Because the engine can cost you already $4000, so buying a already built car for $7000 is not such a bad idea.
If you think about all the time and money you need to invest when building a car from scratch it makes perfect sense to buy a car which is already finished or already started. And you can modify this car to your liking after you receive this car.