I tried to update the theme and got an error saying update package not available. So I downloaded the latest zip and uploaded it, following the update instructions in the documentation, after uploading I got this error: "The package could not be installed. The theme is missing the style.css stylesheet. Theme install failed."
Comments
Anyway you can't use automatic WP update for theme updating. Although you will receive info about new version.
To update( http://apollo13.eu/docs/spaceship/#!/installation_update_update_theme ) them you have to follow steps like in install, mainly in point Download the latest zip file for your theme you should do point from installation process: Download ZIP file from ThemeForest and find in it spaceship.zip(theme file).
So just don't upload whole package for ThemeForest and you will be safe :-)
With regards.