WordPress is licensed under GNU General Public License (GPL), which has several implications to your plugins.
The most important one is that plugin code using WordPress functions automatically becomes GPL as well. This still means you can sell your plugins. But it also means that anyone who buys it or downloads it from the Internet is free to modify it and even resell it without asking for your permission.
A commercially oriented approach to plugin development is still possible, as most clients will be interested in a long-term relationship, with support and plugin updates included.
Technically speaking, there might be a way to license your code differently if you created it in the following way:
Write most of your code as a library, without any WordPress calls.
Create a WordPress plugin that will call the functions from your library.
This way, only the WordPress plugin becomes GPL, and you can license the library separately.
The GPL license is still subject to many different...