A gang of car thieves has been selling stolen motors and then stealing them back from the new "owners", police say.
The thieves keep hold of a key and later steal the car back
Victims have been conned into paying up to £25,000 for an executive or 4x4 car, advertised in a trade magazine.
But the buyer is given only one key to the vehicle. The thieves later use a second key to steal the car again.
South Wales Police said the tactics had netted the gang £250,000. They warn car buyers over the Bank Holiday weekend to be wary of unusual offers on cars.
The force said the car thieves had developed a method which should raise alarm bells with anyone who thinks they are buying a top-end vehicle for a lower-than-average price.
A spokesperson said: "The seller arranges to meet up with the would-be buyer at a neutral location where the car and the cash are exchanged.
"It is only when the new owner tries to register the vehicle with the DVLA do they discover that it is stolen.
"In some cases the car has been stolen back from the new owners."
The force's advice is:-
Never arrange to collect a used car at an anonymous location - go to the vendor's home address;
Ask for the seller's home telephone number and call it to check if it is valid; and have the car checked out for any outstanding HP and to ensure it is not stolen.
Check the validity of the vehicle's registration documents, chassis number, MoT certificate etc.
The force said people should be suspicious of vehicles that come with only one key and they should be careful where they keep their keys once they have bought any vehicle.