I've always looked to buy in winter, three reasons
1. Easier to spot a damp/leaking van
2. Out of season, so I have free weekends
3. Prices are noticably lower and decent vans don't sell as quickly (in summer a good van, fairly priced will always go quickly)
I sold my first van end of summer, no problem. Quick sale, excellent price.
Second van, advertised in autumn only got low offers, didn't sell. Come the spring lots of interest (and timewasters), again sold for an excellent price.
I think the ideal solution is to sell end of summer (good prices, plenty of demand, lots of eager buyers who have been looking all of summer) then get your funds ready for the next van in winter.
My problem has always been finding the right van to buy, not so many good vans out there...