The Ars Technica article above goes on to mention that before the February 2015 reclassification of ISPs the FTC could have punished the ISP's for violating customers privacy. So it would appear repealing Net Neutrality rules would mean ISP's would be breaking the law if they were to share personal data with 3rd party companies.

This can't happen considering you stated our legislative branch voted to allow ISPs to sell our data. They wouldn't be breaching a law and the FCC could not punish them.

The FCC doesn't have the ability to repeal a law passed by the legislative branch.