If its a item vs item trade you can use the trade window to complete your trade safely.
if it involves items that you only allowed to have one per char then you will need to find a way to complete the trade in that game.
Like mentioned before, in D2R, mediators do not come into the game and hold the items for you, just like for example Path of Exile.

The whole trade is done between the others users and the mediators can help with item vs fg trades.