How can ones safe trade through mail?
The simple answer is that there is no way to safely trade with other players outside of the trading post. Outside of the trading post you’re working entirely on the honor system.
Men of Science [MoS] – Tarnished Coast
You can’t and Anet won’t do anything if you are scammed by mailtrading. They implemented the tp as the only reliable source of trading between accounts. If you trade outside of the platform they provide for you it is at your own risk.
There is no safe trade through mail. The only safe trade is through the TP.
ANet may punish the scammer if you lose an item through mail trade, but you will not receive the cash or the item back.
Vayra – Elementalist
Forkrul Assail – Mesmer
I would recommend against trading this way period.
The high value stuff you consider trading through mail where the trading post tax is meaningful is also that much more tempting for people to try to scam you on.
If you really insist on doing so anyway, get your money first. If they’re wary of doing that, which is fine, just decline to do the trade.
You can see if you know a third party that you both trust — a guild leader, commander that you’ve worked with, or a common friend — and each of you send to that person instead of direct to the other.
Sorrows Furnace