Here is an example of such a rule, from StereoNET Australia.
Members posting nuisance replies (bagging of products, off topic discussions etc) will result in their posts being deleted, and possible suspension. You may not make any post which may potentially undermine the sale, including questioning the price, starting rumours about the provenance of the unit, and so on."
This is a simple rule, and it should be followed. Your personal opinions on why a price is too high, what is wrong with a product, why you moved on from it, whether a RRP is inflated, etc, etc, are not to be expressed in someone else's for sale thread.
If you want to raise an issue with a FS thread, consider whether it is appropriate to report the post, or PM the member selling the goods.
Otherwise, please allow members to do their own research, and make up their own minds.
I think such a policy is fantastic and would like to see it similarly implemented here.
Thoughts?