I would look at motorcaravan magazines like Motor Caravan Monthly, lots of used vans advertised. A quick look in the July mag shows for examples, two Duetoes both 1996 ,so old model vans, both asking around £17,000 indicating loony prices private and trade sellers can ask. A 2002 automatic high spec duetto is advertised at £24,000 o.n.o. If you are considering paying around £25,000 for a van, consider also buying New, like imports built on lwb fiat/peugeot vans these can be brought for around £26,000 ( from memory , names like Trigano or tribute) For a guide price of the value of the van you are considering buying, dealers advertise in the back of the caravan magazines wanting to buy. You could try them with a description of your proposed van for a valuation. Good hunting.