Assuming this was a private purchase, it'll have a lot to do with how the car was advertised and what your niece was told. Buying privately is one of the riskiest ways of buying a car because if something goes wrong with it, you don’t have as much legal protection as you would if you’d bought the car from a dealer. You also have to take into account the fact this car is now 16 years old, so it should've been thoroughly looked over before your niece bought it as it's bound to have a few issues, even just from wear and tear. When buying privately (from an individual rather than a business), buyers have fewer rights. In short, private car buyers only have rights if the car isn’t as it was described. The car should be true to the advert and what the seller told you. Your niece won’t be entitled to anything just because the car is faulty or because the seller failed to mention something in the advert.

However, the seller must accurately describe the car, such as the number of previous owners. They must also not misrepresent it, for example not disclosing that it has been involved in an accident or providing a false service history. If the vehicle wasn't 'as described', the buyer can either ask for the difference in value between what they paid and what the car is really worth or ask for the cost of making changes to the car so it matches the description.

Have a full read of what you need to know here: www.honestjohn.co.uk/problems-with-a-new-or-used-c...#

I would also advise speaking to Citizens Advice.