You cannot pay through eBay checkout for a vehicle.
The seller is correct , after purchasing the van from their eBay listing, the seller is allowed to exchange, contact details.
You go and fully inspect the van, if all is OK with the vehicle and the paperwork, you then pay the seller cah or by bank transfer.
Do remember, there is no eBay protection when buying vehicles, so essential to give the vehicle a good inspection before parting with any cash and driving away.
@adam8888_1