Hello Karen, this is mainly a touring caravan forum , but you may get replys, to help your search. The first place would be the site the static van is pitched on as the pitch would go with it. Then possibly a static caravan forum site, then of course Ebay and Gumtree. Good luck to your folks.
A quick Google search has found some websites offering guidance, for example:
It might be easier to sell with the existing pitch, but this will probably require the site owner's agreement to transfer the licence. The site's terms & conditions may also give the site owner a cut of any sale price. Selling it without the existing pitch will require the caravan to be moved off-site which may be difficult and involve additional costs of transport & storage.
I am afraid this may be more difficult than you think but first you need to read the conditions the site attached to the original sale. Most sites make this very difficult or leave you entirely in their hands.
Sites will often make you sell through them and they set the price and take a cut of that or demand it is sold back to them at their price. You will almost certainly find there is a ten year lease on the site and the value of the static is usually dependent on how long there is left on it as at that point they can demand you replace it or leave. Leaving may not be that easy as you need a large truck and the site can block access to that but whatever there is a lot of work involved in disconnecting everything and getting it loaded.
A friend of ours manages a static site and their terms are pretty severe, frankly anyone buying there wants their head examined, but they still buy.
I hope yours is one of the better sites, but until you check the terms you cannot go further.
My in laws had a static at Bidford on Avon years ago. The land owners will only let you sell back to them at stupid prices. As Ray says it will cost you thousands to move it and that assumes you can find a new piece of land to resite it.
Negotiate the best price you can and walk away and avoid the stress.