Ebay is fine I think, there are scams everywhere even on Autotrader, the thing is if you don't give them any money they can't scam you, I always do a vehicle check for a few quid via this app
https://www.carcheck.co.uk/, also at the free site
https://www.gov.uk/check-mot-history
and then you pay when the keys and the van are in front of you, some places will do a physical check if you're not confident to do your own, other than the app I have and always will do my own checks, I trust my own opinion more than a paid one, have had and repaired so many vans in the past.
I prefer to buy privately as no dealers profit to pay, and you get more info on the van and kit too sometimes but have found some dealers who want to deal on a van with problems I'm up to fixing, so I can add value, use it for the season then sell at a good profit, and I have had a good hobby from it.