Do you wanna do a BIG trade that is more then what you can fit in the trade window??
First off figure out what you and the other player agree on trading.
Then both of you, e-mail STS agreeing to allow the trade to be done.
Make sure to e-mail STG the other players e-mail ticket number so it is easier for them to find them.
Then ask STG to do the trade for you!
You will never have to run the risk of getting scammed by a middle man or having one player run off with your items and gold by not following through with the trade.