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The GPL v3 license calibre is licensed under does not restrict use. The short summary of what the GPL says is if you distribute calibre (with any modifications) you must make those modification available to anyone receiving your modified version and the changes must be licensed under a compatibile license (typically the GPL v3 that the orignal is licensed under).
Also, there are a number of commercial entities using calibre as part of their ebook conversion process. Comercial use if fine as long as if you decide to or start distributing a modified version of calibre you comply with the terms of the GPL that calibre is licensed under.
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