I registered a .us extension of a startup which is now public on Nasdaq.
Is it legal for me to sell the domain name on a marketplace like Flippa or Godaddy Auctions?
P.S. My intention wasn't to be a "domain squatter". I registered the name originally thinking I could start a blog (not related in anyway to the startup which blew up in popularity).
If you registered a domain name that contains a name that is protected, then you likely fall within the purview of 15 U.S.C. § 1125(d).
You should consult an attorney, as the consequences for attempting to sell the name could be quite severe. Given the severity of the consequences, it is unlikely someone would try to answer your question without knowing more information and without the protection of confidentiality.
The way you phrase:
My intention wasn't to be a "domain squatter"
Tells me that you already know the answer, so why don't you do the right thing, contact the company in question, explain the situation, and see if they want it. If they do, I'm sure they'll offer you something for your troubles. If they don't, you'll have an easier time deciding what to do with it.
edit: You could also run the domain through GoDaddy, or Domainr to see if this company has a portfolio of .tld variations, or just their preferred .tld - if they have others (eg .com .co.uk etc) then they're more likely to want to scoop up yours.