I don’t know if “rules” preclude price discussions, but if not, accepted etiquette does unless the original poster asks for input. That’s for vans posted for sale by forum members. For vans posted as “found on” 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 it’s price if you posted it for sale? If you think the price is too high then just move on.