I'm guessing as the dealer has seen the van and we haven't.
(If he hasn't seen it then the chances are when he does he will try to screw that offer down.)
If the dealer has offered you £2300 then he would probably envisage putting it up for sale for around £3495 after he has done anything that needs doing. He will also include a 28 day guarantee if he is a dealer and doesn't try to sell it 'privately'. He may also have to 'sweeten' a trade-in deal when selling it from the profit he makes when selling your van. (offering £1200 for a trade in that will sit in his books at £900 say and taking the £300 from the profit he makes on your van).
I would say that selling it out of hand with no guarantee you could expect to get around £2750 - £2950 if everything is mechanically fine and presentable - ie vallet etc. (£2950 sounds a lot cheaper than £3000). However, I would try an advert at a slightly higher price with pictures and see what happens. If you don't get a response readvertise a couple of weeks later. Take the van to another location and take some more pictures so it looks like a new advert. Don't use any of the previous pictures and rewrite to advert so it doesn't bear any resemblance the first advert. Never, never used words like price now reduced etc.
The trouble with buying and selling vans is that its very market driven. For example, if a large commercial fleet user puts several hundred vans on auction then the market becomes saturated for a time and prices drop. Try a little later and you could sell it straight away.
There will always be those who offer a low price, as there is always the possibility that the owner is financially desperate to sell.
Of later I've been watching Mondeo TDCI's on auction on Ebay. I've seen a couple reach higher than expected prices, yet what I would see on paper as a better deal (lower mileage etc) not even fetch a bid a couple of days later.
Spruce