Matt Cutts recently wrote about something that Google is up to.

http://www.mattcutts.com/blog/how-to-report-paid-links/

In a nutshell it says that Google is possibly looking for ways to penalize those that buy/sell links to increase rankings. Many “white hat” webmasters sell links – hey we need to eat and pay for hosting right?

Those links may be less valuable and can also possibly give your site a black mark for a 5 dollar link.  Go read through the comments and make your own decisions.

White Hat Techniques

Blackhat – Whitehat Techniques