Almost half of all motors sold privately turned out to have hidden problems when buyers paid for a car check prior to purchase.
Of these, a third had previously been written off, while around one in five had outstanding finance, which means they could be seized by the credit company without compensation.
Others had been previously stolen or scrapped, making them a danger on the road.
Registration plate transfers, colour changes and imported vehicles were also spotted by MyCarCheck.com.
Company head Roger Powell said: “Buying a used vehicle can be exciting, but the private market is fraught with risks.”
Buyers think they get more value for money in a private sale, but growing numbers are playing safe shifting to dealers and car supermarkets.
“They are attracted by warranties and manufacturerapproved used car schemes too,” Powell added.
You can check your used car purchase for hidden nasties through companies such as the AA and HPICheck.com.
These companies check data provided by the police, DVLA, insurers and finance houses and alert you to any problems.
Costs range from £2 to £20, depending on the information you need – a tiny proportion of the purchase price.
A full RAC inspection is dearer, from £99 to £239, to see if a car is mechanically and structurally sound.