I suppose that would depend on its age and how much you are paying for it.
With our 1997 van I just got a bit of paper where I had written out the amount I was paying for it and the CRIS Number of the van and got the seller to sign it.
------------- Lobey.
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When I bought my caravan, the seller wrote their name and address, the caravan details and price on a piece of paper.
He then signed and dated it and I also signed it before he photocopied it so we both had a copy.