There always used to be a 2nd VIN stamped into the metal of the car in a place that was easy to find yet difficult and expensive to change without making it tampering very obvious. Normally under the drivers seat (flap of carpet lifted up) or on the scuttle under the bonnet.

I have heard that it is possible to slide a piece of paper (or similar) between the screen and the VIM plate to either cover it up (suppose it could work when a total muppet is buying) or put the desired VIN on before its slid under. but then again it might just be another urban myth.

I PX'd a car back in 1984 that was subsequently cloned. Someone (probably at the dealer) copied the chassis plate off my car and riveted it (like they were back then) onto the stolen car and simply had plates made to match. A year went by before someone spotted that there were 2 red XR3i's on the same reg living in the same town. The one I thought was my old one wasn't.

So good advice to all car buyers to protect themselves !

Another way is to depend on cheaper, less enviable cars without bling ....