Can we sale composer package in codecanyon ?
Yes, you can!
I wonder what will be the category. And do we have any composer package for sale already?
It would go under PHP scripts. You’ll want to make sure your description is very clear that it’s a library, not an application.
There are several composer packages for sale including one by me, you can find them scattered among these search results: https://codecanyon.net/category/php-scripts?term=composer