Forum will provide option to lock transaction fg for the duration of the trade.
Player that is about to pay fd will simply sent it to this locked-intermediate state between him and player 2.
player 2 must confirm that he indeed is expecting to receive this amount (in case someone is impersonating him)
Sending player can release fg to the buyer at any time.
if player 1 or 2 did not create a scammer accusation topic - fg is released automatically after 24h (fallback)
if however any of the players made a topic in SA - moderator can extend this block until situation is cleared-up

this will prevent any of the sides to spend fg quickly in order to move it away using runes/items ingame.
additionally it will make more clear who is actually trading who.

That's basically how using mediator works but with extra unnecessary steps. Locking your gold only means you can't make multiple trades in short time.

Use the free mediation service which removes the unnecessary need to lock gold. Here's the guide: https://forums.d2jsp.org/info.php?p=11