SEO gurus should have read already, but just in case you want to be one of them SEO gurus someday and you haven't read of it yet, selling links can harm your PageRank in Google. What is PageRank anyway? It is a patented trademark of Google and is also one of the tools that made Google what it is today. It basically finds out how relevant your site's content are, and based on the need for that content, assigns a rank to your site or page. Or at least, that's how I perceive it. Can I bait anyone into providing a more accurate and precise description of what PageRank does and what it means for SEO? I'll just keep my fingers crossed. Anyway, it has been found out recently that Google is penalizing sites that sell links. Why is Google doing this? Before I answer that, or speculate on it, let me clear something up. Selling links on your site is NOT just selling links, that someone talks to you and gives you money in exchange for a link to their site. You are selling links if you are using a sponsored theme in your WordPress blog, or you get paid writing about something that includes anchor texts linking to a site that pays for it. Now, why is Google doing this? Danny Sullivan wrote a very interesting article about it. There's a link above if you want to read all about it. The gist of it is that it is Google's Search Engine. They created it. It's theirs. They have every right what goes or not. Quoting Mr. Sullivan:

• Google is not telling people what to do with their sites, which is a popular argument point. Google is telling people what to do if they are concerned about doing better in Google. Don't want to be harmed in Google? Don't sell links.
• Don't care about Google? Sell links all you want.

Can you still display ads like AdSense, AdBrite, Bidvertiser, etc? Of course. Those who want to gain popularity quickly and have a high PageRank may have to rethink their strategy as buying links off of websites is no longer an option. At least, that's what Google hopes for. Also, the PageRank decreases happen after human review. I wonder if they're hiring. So why is Google doing this? For World Domination Peace.