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If I have understood the license correctly, all modules made for XOOPS are GNU/GPL licensed, since they do not work without XOOPS and XOOPS is GNU/GPL licensed.
This doesn't mean that they are free of charge, but that if you buy a module from someone, you have the right to get the source code (pretty hard, if you didn't
) AND you have the right to modify and/or distribute it for nothing.
This is of course provided you leave the original credits in the code.
You are not obliged to give credits in the visible output, but are of course welcome to do so.
***Disclaimer***
I am not a legal expert, but I do believe that this is what the license and XOOPS boil down to
***Disclaimer***