I have previously posted 'response' ads pointing out serious problems or misinformation in for-sale ads, such as a bike with obvious but not mentioned fork and frame damage from a front end collision.
But if someone wants to ask $4000 for a rusty Huffy that's just the free market at work. Monetary value is set by what people are willing to pay. I'd like to think people are smarter than that (like smart enough to google a bike before buying so they know it's worth $89 new instead of whatever crazy price someone is asking), but if they don't it's not my responsibility to hold every single person's hand and make sure they don't spend too much.