I donít know if ďrulesĒ preclude price discussions, but if not, accepted etiquette does unless the original poster asks for input. Thatís for vans posted for sale by forum members. For vans posted as ďfound onĒ CL or whatever, not owned by a forum member, usually price and other discussions are acceptable.
Think of it this way: would you want people tearing down your van and/or itís price if you posted it for sale? If you think the price is too high then just move on.