This has been a difficult thread to follow, so it's not clear exactly what has happened. But essentially I the OP believes, he has lost money, then he should be contacting the police.
Unfortunately many of these large caravan sites have their own rules regarding owning caravans on their sites, and in some cases, private owners are not allowed to sell their caravans except to the site themselves.
If this is the case, the owner cannot sell privately.
The OP must seek professional legal advice. And lodge the matter with the police.
I seriously doubt the matter will be resolved by this forum.