For anyone else who might be interested in this, I got a reply from webflow support;
"Templates submitted to the Webflow Template Marketplace must be entirely original. That means they can't be based on any cloned or third-party projects, including the Relume Styleguide. Even if customized, it would not meet our submission guidelines for marketplace approval.
To successfully pass Webflow Marketplace verification, we recommend starting from a blank Webflow project and manually recreating any styles or class systems you'd like to use (inspired by Relume or Client First is totally fine!).
There鈥檚 no restriction on what you sell externally, you're just not allowed to market it as an official Webflow Template or use Webflow鈥檚 brand to promote it, unless it鈥檚 gone through our Marketplace approval process."