Is it fair to use an MIT licensed jQuery library inside a plugin and then sell it as a freemium plugin on WordPress.org?
I would normally not answer, but since I answered your other question once please allow me to.
If you check the MIT licensing model you may read:
MIT licensed software can be integrated into GPL software, but not the other way around.
Since WordPress themes and plugins should be GPLv2 I would say it is possible and legal.