Sorry I have an issue with the usage of the slang.

According to dictionary.com:

noun Slang.

narcotics officer.

Narc as a noun means a narcotics officer.

I think the term you are looking for is a Rat:

Slang.

a person who abandons or betrays his or her party or associates, especially in a time of trouble.

an informer.

a scab laborer.

Specifically you will be informing.

Would I be a narc rat for sending an Airbnb listing to my condo board?

Yes you are a rat, but I would not use that slang as it is negative.

You should call yourself a "Whistleblower" instead. This has a better connotation:

a person who informs on another or makes public disclosure of corruption, wrongdoing, problems, or secret information, especially within an organization.

If you are whistleblowing you are joining the ranks of Edward Snowden, Chelsea Manning or Dr. Jeffrey Wigand (Portrayed by Russel Crowe in the Insider).