A few things to note is that all other sites listed there have either disappeared, only offer the products under a subscription, at cheaper than the developers rate or force you to donate to be eligible for downloads.
I will also post here some comments this articles author found from several plugin developers in case you don’t want to visit the article yourself
“The act of copying my themes, making no changes, and selling them is perfectly fine. I gave you permission to do so by placing it under the GPL license. Just in case that wasn’t enough, I’m giving you or anyone who wants to do so permission right now. It is not unethical for you to do these things so long as you do them within the confines of what’s allowed by the license.” – Justin Tadlock
“Licenses sold for WPtouch Pro are for support + product updates only. No fees are charged for the GPL software license and its freedoms attributed to this software.” – BraveNewCode
There is also a video showing Matt Mullenweg (he developed wordpress if your not sure) discussing the GPL and its ‘ramifications’ on the article.