It must be having some special algorithm for that too lol...... sometimes its obvious on the site that the link is sold as it might be of a different niche
Very good question. My answer is No, in fact google give you more respect if you did. See yahoo directories, business.com and other directories.
if you are a buyer, i think google won't know about that...except you call them . But if you are a seller, they will know that, because they always crawle your site. See how many outbound link and is it normal or not. The danger one is blogpost,,,i am really don't want to accept buyer to place their post on my blogs if they have to many link on post and not related to my content...
google spiders are too smart no sense buying links they never do any good anyhow. Do do follow blog comments thier easy to do and just take some time.
Guess they know from the link scheme you make. If the scheme is repeated in the same manner, then you go for less value.