Let me guess, the notice contained a link to change your PW, which you clicked on, right? My guess is you've been scammed and someone now has access to your account.

This happens frequently with things like Facebook, Hotmail, Gmail, etc.. You'll get a legit-looking notice such as you did and they'll include a link to take the action on. NEVER click the link. ALWAYS make any changes from your "account settings" in the app itself.

What you should do RIGHT NOW is go into your Facebook account settings and change your PW there.

As for your initial Marketplace issue, it was probably legit. But Marketplace have some rules about certain words and an ad for a "paint gun" could get flagged and cancelled because of the word "gun" (as an example).