Sorry if my replies sounded angry. I don't think you're going to run into any licensing issues unless you're doing something really unusual in your business model.
I (like thousands of other people) earn a living developing websites in WordPress. There's absolutely nothing wrong with selling the themes and plugins you create or charging for your time to train users or setup and maintain websites. You could even charge a monthly fee for hosting, backups, security services etc.
The only thing you can't do is charge money for the WordPress core or the MySQL software. But since they're available for free online, it would be pretty hard to find a customer who would be foolish enough to pay for them anyway.
There's nothing stopping you from building a theme or some plugins and charging your customer $5000 or whatever for the software. Heck, if you can find a customer who's willing to pay $1 million for your themes, then you're certainly allowed to do so. Charge for whatever you like as long as the WordPress/MySQL files are included for free... or not included at all.
The main point is that WordPress, MySQL, and any third-party plugins remain the intellectual property of the people who create them. That's pretty much it.
Because you're the author of your themes and plugins, it's 100% your choice whether you sell your customer ownership of your code or if you only license it to them for one specific project.
Does that clear up your concerns?