The Police National Computer (PNC) is updated whenever a vehicle is reported stolen - that's 160,000 times a year - and sends these updates to AutoCheck daily.
AutoCheck searches for PNC details of the car you're interested in. If it's there, we'll alert you and contact the police force that recorded the vehicle as stolen to find out the current situation.
You should never buy a car if it might be stolen. The original owner (or their insurer) is entitled to reclaim their vehicle, even if you bought it in good faith. Your only option would be to sue the seller - if you can find them (which is unlikely, given they are selling stolen vehicles).