My two cents on this issue are similar to Grey's but also John's.
Negative slander or "I wouldn't purchase this nihonto" should not be allowed. But some commenting on overpriced items should, even if the normal rule of doing business is "It is not stupid to ask, but to pay". John gave a valis point on the integrity of these forums as if we do not allow any kind of commenting, we might see an influx of people coming just to sell their wares here as these forums are regarded as trustworthy and thus it can be seen similarily on the sale section of these forums.
But where to put the limit and which items can be safely said that are a bargain or otherwise? This might pose a difficult question to answer. Should we limit the comments only on more experienced peers or some other manageable way to make sure that comments are not just "opinions" or sales pitches?
Sorry, more open questions than solution proposals..
Antti