Nothing wrong in giving valuations at all.
If the item is overvalued then it may not sell when the vendor puts it up. If it does happen to sell at the higher price then clearly the 'overvaluation' was correct after all!
If someone undervalues an item in response to a request its unlikely to do any harm (to the vendor) because there is practically always more than one response anyway and a vendor would be a bit foolish to base his price on the opinion of just one person.
My main concern about valuation threads is that despite the mods' vigilance I suspect it likely that at least some requests for valuation are simply attempts to auction the item. How many times do you see a request for valuation, but the gun never goes up for sale? Chances are that's because someone has pm'd or emailed the seller and bought the item that way. Nothing wrong in that (apart from breaching a BBS rule in a way that makes it impossible to enforce) but to my mind it is against the spirit of the BBS.
By giving realistic valuations we may help to stop that happening to some degree but there will always be someone willing to pay over the odds for something - e.g. for sentimental reasons.
Happy Shooting!! Paul.
"We cannot solve our problems with the same thinking that we used when we created them" - Albert Einstein.