I had this a few years ago with a set of ko radio. Won the item,it had no reserve, won it for about £90, like you recived the emails from ebay saying you have won the item. I then recived an email from the seller were he said he was not prepared to let it go for that amout and if I were to double the winning bid he would sell! I sent an email back saying he should have set a reserve, and I had won fair and square and was not paying another penny, he then sent his phone number, so I rang him, He said he was unable to sell for that amount and that he was unsusure how ebay worked! yet he had 20 feedback, so he had been doing it a while. I never paid, he never sent the item! Ebay un intrested realy.
But in your case it's a much larger sum of money, Like has been said, you have bought the item and sould be able to start legal procedings, but thats loads of hassle, prob better to find another van.Leave him the worst feedback known to man!
Thing is on big items people try to list the item for as little as poss, thus they don't want to pay a reserve price fee
You could arrange to go see the Van, It may be a stunner and still cheep at 5000,and worth 6000 As you have been polite all the way through you may be able to hammer out a deal at 4750 or somthing,As you have the leverage of already winning on ebay, and I suspect they need the money, You will also be able to work out if it all genuine or not