Most products available on this market place are free of charge and can be downloaded directly from this market place. No payment in required in this case.
However, some authors decide to charge something to distribute their works. There is no problem to conciliate both approaches (Price to distribute code versus OpenSource licence): The Free Software Foundation, that has created the GNU GPL Licence, one of the most used licence for OpenSource software, allow and encourage this, and provides some explanations on this page :
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