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I drastically modified an free extension to do much more than it does and feel I should profit from it by selling it. Before I modified it, I contacted the extension owner several times to ask them to modify it as needed and I would pay them for it, but got no response.
Does anyone know what the rules are in selling a highly modified free extension?
To answer this question requires knowing the license terms governing the original module.
Did the original module come with a license agreement (perhaps a License.txt file in the module’s zip/gz archive)? Do *all* of the modules’ PHP files contain a text header saying that GPL or OSL or some other license covers the module?
When I click “Get Extension,” a link to the extension’s license agreement is displayed. Take a look at that agreement. While an understanding of the entire agreement is important in answering your question, section 1b is particularly of note. Additionally, the “A brief explanation of this license is available” at the bottom of the agreement may be helpful.