The two main things to watch out for when buying privately are the year of manufacture, the caravan is often advertised as being manufactured during a later year thus causing the caravan to appear 'younger' than it actually is and of course damp / water ingress which is the biggest killer of caravans.
All caravans manufactured after 1992 are recorded on the CRiS database (More information
Here) and the owner should have a copy of the registration document. The CRiS number will be etched into the caravan windows in the bottom corner, More
Here on how to read CRiS NUMBERS. ( The letter for 1999 is X)
People selling used caravans will swear on their granny's life that their caravan for sale is as 'dry as a bone' but never take their word for it.
Take a small hand torch with you when you look at the caravan, open cupboards and look under bed lockers and into corners for signs of damp, bumpy wallboards, soft spots, black mould etc.
Look behind curtains, tell tale black mould often shows on curtain linings and gently pull back window rubbers to see if there is black mould or gunge underneath the rubber.
Have a look through the Caravan Buyers Guide,
Click Here
The guide contains a wealth of information for a used caravan buyer, click on the oblong buttons down the left side to read the different sections.
Good luck and if you have any questions at all just ask