No serial number is required. No activation is required.

Technical Conclusion
This module:
is commercial and large
has a lot of code, which might be alarming
but doesn't exhibit any malware patterns

Everything detected fits with:
“Marketplace with vendors, payments, emails, and licenses”

Recommendations (best practices)
Although it's clean, I recommend:

Correct permissions:
/modules/ets_marketplace/ → not writable

Keep it updated

If you suspect something in production:
review email logs
review outbound traffic (firewall)
