Very simple - here is what you do: Open trade with the gold buyer. Put the amount of gold into the trade window. Wait for them to hit "trade" turning their side of the window green. Then, hit trade yourself turning your side of the window green. Now you have about a second to take a screenshot. Following this, take another screenshot when the words "Trade Complete" flash across the screen. Send these two screenshots to the representative you are on IM with and you're done.