Can I use a theme done for the previous version in the latest core version? Is it possible/advisable to use a Drupal 7 theme on a Drupal 8 installation?

I'm starting my first Drupal 8 project. Generally, I hit up a place like ThemeForest for a theme, but their D8 theme selection is depressingly slim. Can I use a (highly rated and actively maintained) D7 theme instead? Is there a better place to look for professionally built D8 themes?
No. You could convert one if you have the time - which involves turning all tpl files to twig, updating the syntax, and making necessary changes to the template.php file (which gets renamed to template.theme). That also includes replacing any functions that don't exist anymore, and using the new D8 equivalent. Again, not too hard, but it can be if you are brand new to D8.

For smaller themes, this can take few hours to a day or two; for more involved themes, it can take longer.