Older accounts (2015 and below) with at least 3k posts and more than 1k fg are trusted in my eyes and you can go first for low and middle value trades.
For high value trades I suggest mediator assistance. How much fg is a high value trade is up to you.
How to request mediator and the mediation process.As a new trader you have to do due diligence. Check
Scammer Accusations for recently accused accounts, it is also an
excellent place to learn common mistakes and scam tactics.Pay attention to offers you receive from personal messages, almost all of them are scams (one way or the other).
Ignore all whisper offers you get from bnet accounts, tell them to contact you on d2jsp for d2jsp trades.
It's a lot better to miss a trade than to get scammed, don't let scammers pressure you into a "trade".
If they don't have time for a mediator trade, but still refuse to go first, let them walk. Especially if it's an account with 100-300 posts more than you and low fg / join date.
If the offer seems too good to be true, it's probably a scam.