WordPress rocks!
I have been using wpthemesbox.com for a long time. They have developed a great system. They sell 100% original licensed themes and plugins. Thanks to the plugin they developed, I can install it on my website in 2 minutes after purchase. It also receives automatic updates from the developer. I use it on many of my sites. However, they only sell popular themes and plugins.
You're absolutely right to be cautious when purchasing themes and plugins from websites offering very low prices. While some may seem like a great deal, as you pointed out, they can often be 'nulled' versions that come with serious risks like viruses, trojans, and other security vulnerabilities. Using such products could put your website and customer data at risk.
"Often" you say? There's definitely some sketchy sites out there, especially ones that a free. You would be surprised how many plugins and themes popup on malware scanners that come straight from the developer. I had some themes and plugins that used pollyfill.io to load JS for older browser support. Somebody ended up purchasing the domain and used it to inject malicious code into anything that was still calling it. Really Simple Security is another plugin that fell victim to a vulnerability. My advice, use as few plugins as possible!
