Can we sell the product (As the know how to use) after customized for the customer requirements?
I dont think you can sell a free open source community driven program - Sure you could sell support, i have seen others do that.
Perhaps I’m reading this wrong but it looks like I could take LibreNMS and sell it if I wanted to:
When we speak of free software, we are referring to freedom, not
price. Our General Public Licenses are designed to make sure that you
have the freedom to distribute copies of free software (and charge for
them if you wish), that you receive source code or can get it if you
want it, that you can change the software or use pieces of it in new
free programs, and that you know you can do these things.
I would imagine that many companies charge for support because it would be kind of trashy to charge someone for LibreNMS when it’s free. If I charged you to install and configure it and support it, that would be different though.