Bad condition
Should your car suffer from poor bodywork, a worn and torn interior or rough mechanical conditions, it might be worth getting all or some of these checked out for repair. They all play a role in decreasing the value of your car in the eyes of a new buyer.
Previous owners
The higher the number of previous owners, the less likely others will be to buy your car. If there are several previous owners, questions start to be asked about why the car has changed hands so frequently. It's also more likely that the car will have poorer quality or missing service history.
Higher mileage
Having higher mileage than most similar models of its age counts against your car's value. It's worth bearing this in mind before it happens. Though, if it's unavoidable, be sure to look after the car's repairs, and keep the service history up to date and complete.
Increased age
If your car is an older model, it will usually be less desirable than the newest and improved models available on the market. There are some exceptions to this, perhaps they have desirable qualities that have been replaced within the newer model, but it is the general rule.
Patchy service history
Missing or inconsistent service history doesn't paint a reliable picture of a well-looked-after car. Keep this in mind when buying any car, as it will impact resales. If it's something you can rectify, such as contacting your local garage for a receipt from some past work on the car, it's worth doing.