Against my better judgement I sold something online to someone who turns
up to be a very unsavoury person.
I had had a bad feeling when we communicated online so arranged to meet
him in a supermarket car park for the exchange. I never gave away any of
my details (name, address etc), and when it was all done I intentionally
went into the store do to some shopping.
When I left some 15 minutes later I realised that he was still sitting
in his car in the same place where I had last seen him. I pretended that
I forgot to buy something and went back in, stayed for a little longer,
and when I left his car was gone.

When I drove away I suddenly noticed that he was positioned in the far
end of the car park, near the exit, and I am 99.99% certain that he took
a photo of my car (+ number plate).

I wasn't doing anything wrong, and was honest throughout, but as the
item was 2nd hand I have a bad feeling that he was the type that may
well expect a "lifetime warranty" to come with it...
The item was only a few tens of pounds, but I got the feeling that for
him this was a substantial amount of money.

Is there a legal way for a member of the public to get hold of your
details through your car details?
What about an illegal way? Can you ask your mate the policeman for "some
help"?

Any help will be much appreciated.

PS: he did mention that he was in the car business (mechanic I believe)
if that makes a difference.