Shaddows - 3:11 pm on Jun 7, 2012 (gmt 0)
Have you asked yourself what's stopping Google from simply discounting suspicious links?
Yes. I've posted about it, I'm not sure it will be published though.
But the short answer is the same reason shoplifters aren't politely asked to put the stolen items back. There's no deterrant and no justice for those who play by the rules.
Far more people lose out to rampant competitor link-buying than they do to hypothetical negative SEO.