For gold sales the proof could just be a pic of a bank worth a lot (simply proving you have lots of RSGP to trade if you are selling RSGP). If you are buying it, it should be a screeny of your paypal/other payment method's balance. Basically just proving that you have/had the means to go ahead with the purchase. Make it that it only had to be done within a week or so - so as to not be too much of an annoyance to large traders.