You said £15,000 I thought? That advert says £11,700?
It sounds way way too cheap to me and I would not pay any money up front before seeing the van and checking it over, no least to check it exists.
I would also pay some of the money over if buying on a credit card. I believe the rule is if you pay for goods with a CC transaction of more than £100, the CC company become liable for the entire amount if there is a fraud issue or similar? don't take that as 100% correct and check the details but there is some advantage to you do do it that way.