If your selling package contains campaign and other related content, made by only you, or, you own copyrights to this content, you can sell it no problem.
But you may not include any content, which you do not own copyright to.
In your example, you can sell missions, which you made, but the package should not include original game and/or mods, made by other people, for which you don't own copyright, even if they are required for your campaign.
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