Buying a second hand caravan privately

I'm not thinking about the checks you would do on the van here such as chassis, tyres, damp etc.  I am thinking about the checks you would do on the person selling. 

Presumably an HPI check?  Checking the reg docs against the CRIS system?  Apparently this chap has all the original manuals/docs.

Is it fair to ask the seller for evidence of identity - ie photocard driving licence?

Can anyone suggest what other checks one should make?