It seems Matt Cutts the man from Google has spoken and offered some less vague advice for webmasters selling links. I will sum up what he appears to be saying and leave you the links to the sources so you can decide for yourself.
They confirm that Google is punishing sites that sell text links that pass on page rank. Google is not too concerned about graphical banners which also link to the advertisers sites. They claim they can detect these.
Google Webmaster central on paid links.
A discussion on Google groups about paid links.
Matt himself talks about paid links.
If you want to sell links you need to make it obvious to the search engines that it is a paid link by using the nofollow tag. You can also use javascript which is not that great for single links. You can also use nofollow and 302 redirects via an intermediary page to stop search engine bots from following the links.
The good news is that there are a few link management/selling scripts and services that are appearing who conform to Googles nofollow rules. I’ll be covering these in the not too distant future. I am also working on my own custom solution.
