Is there any way for newbies to build reputation besides spending months to become a regular? Maybe some kind of a guarantee given by an already reputable user?
Yes, you need to be an exceptional contributor to the forum, for instance:
nullius Check out their previous posts to see the quality they brought to the table. Whether you agree or disagree with what they say is a different thing, but they did build a reputation here quickly with the knowledge they shared on the forum.
Yes, I will get a guarantee from a reputable user if they are willing to compensate for the financial losses you made to others on the forum. Meaning, if I'm doing a trade with you, the reputable member would be putting his money in escrow should things go south.