All Cars Run On Used Parts
-- automotive axiom
A used car is a car that has already had an owner. When buying a new car (one that has not been driven off the ) it depreciates by approximately one-third as soon as you take ownership. While new models are only available new (surprise, surprise) you can save a veritable bundle of money by purchasing a car used.
If you want reams of information on actually purchasing a used car, you should check out Buying a used car - it's packed with good ideas. Instead, I will describe the typical issues and condition of a used car.
First, we should consider the things that happen to any car just by driving it. The most immediately obvious repercussion is paint damage; as you drive down the road you will encounter sand, gravel, rocks, bugs, and other small airborne (whether ballistic or windborne) particles that have negative affects on paint. Smaller bits will cause tiny scratches that are invisible on their own merits, while in groups they contribute to a general dulling of the paint finish. Larger rocks will cause paint chips, glass chips, glass cracks, dings in metal parts including grilles and bumpers, and so on. While scratches can often simply be buffed out, chips and dings require fairly involved repair. The smallest professional repair for the tiniest chip or ding will occupy an area of at least a square foot. Even the window glass may be generally covered with minute scratches, although they are easily buffed out using an abrasive rubbing compound followed up by a glazing compound.
As for the interior, generally speaking anything ten years old (or older) will have rips, seam separations, et cetera in the fabric, and the dashboard will very likely have at least one crack. Window seals will be starting to harden, which results in increased wind noise and a poor seal against water intrusion. Every crack in the car, whether it's the seam between parts of the dash, or the edge of a window control panel on the door, will be filled with a fine mixture of grit and dust. If there is a cabin air filter, it will be covered in pollen and dirt. If the car has been smoked in, especially if the ashtray has been used (as opposed to the cigarette butts being turned into litter, or a smoke-trapping ashtray being used) then it will have a strong, distinctive smell of trapped cigarette smoke that will never go away once it has set well in.
Some of the major engine components are actually prone to fail around that time as well. Every 60,000 to 100,000 miles depending on the vehicle, one is supposed to inspect and possibly replace the timing belt(s) on vehicles which use them. Compression should be checked, because with mileage comes wear, and you need to know that the valves, rings, and the cylinder bores themselves are within specifications. A compression test also checks for a failed head gasket, which is common on significantly older vehicles. The power steering pump commonly fails, especially on Ford vehicles; 1980s to 1990s Fords in particular tend to share the same lousy pump.
The suspension has typically also taken quite a beating by this time, and anything twenty years or older will almost certainly require replacement of all flexible suspension parts. Typically, at the end of almost any link between components (except at the end nearest the wheel) there is a bushing that allows some play in the suspension, in an attempt to prevent sharp jolts from causing permanent damage to critical components. Every single one of these will be getting sloppy, leading to inconsistent handling. If the vehicle (for example) pulls one way during braking, and the other way during acceleration, the culprit is almost certainly a bushing in the front suspension. Replacement of these parts can often be the most time-consuming and expensive job which needs to be performed on the vehicle, and often requires the removal of many parts and the use of a press.
Electrical systems are usually in good shape, but some of the electrical components are usually aging, and placing additional load on it. For example, the motors that roll windows up and down (in vehicles with power windows) are usually having to work overtime, either due to internal resistance due to worn brushes or commutators, or due to the grease on the window mechanism getting old and gumming up. This will cause premature destruction of both fuses and switches. This is even more common in the case of motorized sunroofs, since they are exposed to the widest range of temperature variation.
As for the drivetrain, the part of the vehicle that transmits power from the output of the engine to the wheels, the transmission often is worn, and the differentials as well. Wheel bearings also often go out, and in the case of front wheel drive vehicles or vehicles with rear wheel drive and an independent rear suspension, CV joints on axles are often failing as well. In a manual transmission, the synchromeshes are what most commonly fail, making it hard to get into gear even when the clutch is working properly, and in an automatic transmission, the clutches fail and general gumming up of the transmission can impair its operation.
With all that said, you might wonder if the monetary benefits of purchasing a used car are largely mythical. They frequently are, because many people will not divest themselves of a vehicle until well after it should have been removed from the road, but there are certainly those times when people purchase a car, decide it does not fit their needs, and proceed to resell it. The bottom line is that all cars run on used parts and provided a vehicle is not abused, and is well-maintained, there is no reason not to purchase used.