Who’ll is the first one to upload a Drupal 8 Theme?
Who knows, it’s tough to say! I’ll have a Drupal 8 version of our Dawn theme ready within the next week or so, along with a Drupal 8 compatible slider module on CodeCanyon.
Most of the Drupal authors here on ThemeForest rely on many modules for their theme to function as advertised, so it may take some time before we start seeing a lot of Drupal 8 themes for sale. Once the contrib modules on drupal.org are updated I’m sure more themes will start popping up. 
I already did, but that got refused as the design of inner pages was not good enough. If anybody wish to partner, we would be glad to help develop.