I don’t know why Google hasn’t done this before, but it would be incredibly easy for Google to reduce its paid link spam. As we all know, Google is the master of FUD and has thus created a culture of fear amongst link buyers and sellers. Google and/or Matt Cutts could reduce the number of links being sold by sending out a friendly message to the link sellers through webmaster tools and through the contact details on their website. A friendly message might include:
“Dear Webmaster, we noticed that you are possibly selling links on your website. As you might know, selling links is against Google’s guidelines and you risk having your site penalised or removed from the index completely, this would result in your website not showing up in Google and you getting now traffic at all…”
Now, most people getting this message will **** themselves and would probably remove all paid links straight away.
There you are Google; your index now has far less spam than it did previously :)