In some states, a car may receive a rebuilt or salvage title if its owner has replaced a substantial component with an aftermarket part. The parts in question are often large items that the car requires to run properly, like the motor, cowling, floor pan or other substantial part of the bodywork.
An aftermarket component in the car of your dreams may not be a bad thing; if you're a car enthusiast and want a car with certain upgrades, it may even be a major selling incentive. But aftermarket components can carry risks for car buyers: Some could affect the car's insurability, while others could impact its safety.
Some insurance companies don't like to assume the risk that comes with a car modified to go excessively fast, or altered to perform in a way other than it was intended to by design. This could mean substantially higher insurance rates for your dream car. Alternately, states that test car emissions may forbid a car from being registered if it has aftermarket engine components that allow too much noise or greenhouse gas emissions. And components installed by the car's current owner may only be as good as that owner's mechanical skills -- you should understand the modifications well enough to determine if the owner installed them properly before you even think about buying the car.