Lets say the original price inc VAT was £40,000. If the purchasers were VAT registered, they would claim back £6,666 of VAT. If they then sell it for £30,000 inc VAT, they would have to pay £5,000 VAT back to HMRC. You can't have all the VAT back off the original purchase, just the VAT off the £10,000 depreciation. When the dealer who buys the van sells it for say £27,000 plus VAT, he will have claimed the £5,000 back from his purchase price, but will be then liable to pay £5,400 on his next VAT return to HMRC.
This whole cycle carries on until someone buys it, but doesn't claim back the VAT element. Then, the van is no longer VAT qualifying, and VAT cannot be charged.