On our previous build, I sold the static 'van on Gumtree.
A private seller is at a disadvantage, because of transport. Up here there are only 2 people capable of moving them, and they are the two local caravan dealers. One flatly refuses to move vans not sold by them. In fact when I sold ours, the buyer had to get someone from much further away to move it, as the local guys would not play ball. On the other hand, buy one from (or sell one to) a dealer and they transport it for free.
So find out what the local dealers will offer you and make that your starting price on ebay, or ask a bit more on gumtree.
There are two dealers near here that both have a yard full of them, anything from quite decent ones recently off a holiday park, to old wrecks that they sell for £500 as site offices or storage.
The one we have on our new build was actually offered for sale ON a site. I wanted it because it was such an unusual layout (centre lounge, bedroom at each end) and they agreed to take it off the site and deliver it to us.