Advice service Consumer Direct has seen complaints about used cars bought from dealers rise by nearly a fifth from last year.
According to the Office of Fair Trading (OFT), more than 38,000 complaints were registered during the first half of 2010 - a year-on-year rise of 18%.
Buyers of used cars spent an average of £425 on repairs that dealers were supposed to have made before sale, the study said.
Motorists looking to buy second-hand cars are now being advised by the OFT and Consumer Direct to make sure they are aware of their rights.
They are being offered a checklist to go through before making the purchase, including questions about the car's mechanical history, mileage checks, servicing, modifications and the number of previous owners.
Consumer Direct operations manager Michele Shambrook said: "Cars are an expensive purchase, so before parting with any money people need to know exactly what they are getting and what they can do if things go wrong."
Complaints relating to second-hand cars continued to top the list of calls to Consumer Direct, above mobile phones and TVs.
Around 3.6 million second-hand cars are bought each year, with consumers spending a total of £24 billion.
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