John, I personally don't think link buying is dangerous. As long as the SE's don't know you have bought it. I mean, how are they going to know if a link I include within my post is a legitimate reference to a site I like OR if I was paid to post that link. It's almost impossible to detect when done wisely. Now, however, SE's are definitely smart and links found in link marketplaces might suffer.
For example sites that list the marketplace (sites where you can buy links)..it's totally likely that the sites listed there will have discounted value.
But, if you are talking about links from let's say the V7N contextual..it's almost impossible to find out if the link is paid or not.
That said, the other important thing IMO is also the nature of these links. Do they alll appear in the footer? Do 200 links appear miraculously in 2 days? Do all of these links have the same anchor text?
My next point is...if buying a link will get you tons and tons of prospects that buy your product (more than search engines will ever supply me with)..I'd definitely buy that link even if it means google hating me.
My $0
Cheers,
- Ana