There's a great article posted earlier today on the SiteProNews.com site. The article, written by Bill Platt, discussed the recent controversy surrounding paid links and some comments on them made by Matt Cutts. While you may not gain great insight into the buying or selling of paid links (it's not that kind of article) it does lend some interesting analysis of the issue and how the engines (mainly Design) are addressing it.
The article also provides some great links to forums posts discussions of the topic including forums that Matt himself is commenting in. You can read the article on the SiteProNews.com site at http://www.sitepronews.com/archives/articles/2007/0425.html. I'd put this article in the "recommended but not mandatory" category of reading. There aren't any "how to's" but if you're engaging in paid links as a buyer or a seller it's always good to know what the feelings of the engines are and what they can and cannot do.<