Show the seller comparable vans from a nationwide search. Craigslist search aggregators are your friend here. Then give them time to think about it.
It'll depend a little on why they are selling, but if they are moving on from the vehicle, they'll come around to market value. If they are selling because they think they can get a certain amount of money out of it, they will not, but the unit is also unlikely to sell.
'99 EB ex ENG KSWB news van, low rent 4x4 conversion (mostly fixed by now), home built interior.