Even if well cared for, a vehicle may need something. Test drive the vehicle yourself, then have a qualified mechanic check the vehicle. It is the most common guide of a fair price to pay. Most financial institutions use the NADA Guide. It can also give you an idea of value of your current vehicle.
Condition is more important than mileage.
A newer vehicle with 100,000 miles can have the same wear as one with 50,000 miles. Ask if service records are available. Many items may have already been replaced.