Just to be clear, Google doesn't "ban" sites that buy and sell links. (Even Google doesn't have the power to ban a site.)
What Google says is that sites that sell links (not sites that buy them) are in violation of Google's quality guidelines, if they do so in order to pass page rank. Even then, they can't stop the site doing this, nor do they remove the site from the search results.
Here's what they say about this:
The consequences for a linkselling site start with losing trust in Google's search results, as well as reduction of the site's visible PageRank in the Google Toolbar. The consequences can also include lower rankings for that site in Google's search results.
So the worst that happens is that the site will rank lower than it would do otherwise.
However, there is a simple defence against this: add a rel="nofollow" attribute to the link. The site will then no longer be passing page rank, and so will be acceptable to Google. That is no doubt what Yahoo is doing.
(Source for all of the above.)