I did in the past even 20b+ trades with no GMs. And more important : never got scammed a single time.
Usually, first of all you will ask to see the card/gear.
Check on ropd/wp forum/irowiki (if possible) who is that person.
Always main to main trade.
And then, before the deal, you can type in chat something like (i'm usually doing this every time) : "Today, 25 August 2013, @ 2pm PST timezone, i'm buying a Kiel card for 25b from ????? Agree ?". Then you will start to deal 1b,2b,3b,4b... and take screens of the whole trade. Then keep saying 1b/25b given right ? Yes. 2/25b given and so on.
Finally In the last trade 25b/25b (last 999,999,999z) and Kiel card and "Full payement received right ? Yes. Ok now the kiel card is mine. Agree ? Yes."
Always worked this way.
Edited by meditation, 25 August 2013 - 11:45 AM.