Currently dealing links on the "black market" is one of the major and essential tactics of modern SEO. You build a fantastic site, optimize for the search engines, be creative, try to get as much links as possible then buy the rest to give you that "push" above the competition. Although Matt references how they won't enable you to, I believe a great deal of this is wishful pondering, or a prediction from the near future. Currently as long for the reason that advertiser doesn't name your link which has a title link "advertisement, sponsor, link partner" for example, the link will still count, especially if they will include it from the body of their quite happy with normal anchor copy. Of course you'll better be prepared to pay a pretty price for that service, because they'll sometimes be sending you some of their traffic as well as promoting your product instead of just providing any link for SEO.
One interesting matter to note through Matt Cutts. He recommends this rel="no" tag. Again for reference this is the tag that it is possible to add to your links to get them not spread pagerank. Now your first question may be "why would anyone desire to stop a link from passing pagerank or relevancy".
There are a lot of creative answers to the one, but at this basic level i thought this was designed for writers who could reduce the power of links covered within blog comments to be able to discourage comment spammers. Matt Cutts endorses adding rel="no" in order to any links an individual sell. Now if I used to be going to sell a connection to a poker site or a bad neighborhood. This might be a good idea as to not hurt my relevancy in google, but if an online site had a related topic, and was very legitimate, I'll get a lot more money and they'll get a lot more benefit from a normal intact link, and if Post hide it correctly, google will never be able to tell It was taken care of it. Especially if for an additional fee of course, I added any page to my blog or to my site describing what one other website was and why I was linking to them, but that's all hypothetical.
Summary of two different SEO difficulties mentioned here:
1. Google devaluing web page wide links (links on the same domain or maybe website)
2. Google devaluating given links.
