Help! Questions about scammer & stuff (+nonsense celebration GA)

To report, I think you need to register on their forums. At least I recall that my Steam login was not enough to access that part of the site.
After the registration, there is an entire subforum called Report a Scam. One of the stickied topics is their guide on how to do that. Essentially, you need an accused account (the ID64 URL, so the one with lots of numbers), a wronged account, and all possible evidence of the scam.
Keep in mind that they are understaffed as well, so reports can take some time to get processed, and without enough evidence to make the case absolutely clear, the report may be declined and filed without penalty towards the scammer. Just remember that if the value of an item is over two dollars, then you should make screenshots of the conversation.
And always, always check the real trade rep and scam report status of a new trade partner through Enhanced Steam. They can link anything in chat, but that plug-in will show their real feedback.

Confirmed scammers (and even users under investigation) have an eye-catching warning text on their profile (slightly under where VAC bans warnings are located).
Then there're the links to other profiles, including Steam Gifts, where one can check a user's trade reputation here.

A very common tactic used by scammers is impersonating renowned traders, copying their vanity URL, and linking their Steam Gifts feedback page. Enhanced Steam helps prevent this, as it uses the unique ID64 URL to identify users.