As directed from DO twitter reply https://twitter.com/digitalocean/status/1103858313022844928, submitting a entry here.
The DigitalOcean Marketplace docs at https://marketplace.digitalocean.com/vendors outline using img_check.sh tool to ensure image is ready for submission approval. However, img_check.sh makes certain assumptions during the checks which don't hold true for CentOS always. I made a pull request for fixes to img_check.sh to workaround these false assumptions at https://github.com/digitalocean/marketplace-partners/pull/35 and posted issues at
In addition to that, another false assumption I see is for CentOS detection, your img_check.sh script assumes both CentOS 6 and CentOS 7 use firewalld, while firewalld is only use in CentOS/Redhat 7 and not CentOS/Redhat 6 which just uses iptables directly.