(1) The core problem are professional "free-riders" -- Linux distributions being the main candidates. As soon as they include his software in their repositories, his sales will probably stop to increase.
(2) For Linux distributions, it's also hard to make ready-to-go packages. Looking at his download page, there's no sign whether his package installs properly. Can I even use it? Is it RPM or DEP? Or some sort of .run package that needs to be made executable, first?
(3) Expensive software is unlikely to be sold under an Open Source license: think dental office billing systems, for example. Everything more expensive than, say, $25 will make people get the software elsewhere or use the source.
Nevertheless, I agree that selling open source software may provide some benefits for the seller.