The chances of that happening are slim - most users who buy through PA are legit traders.
As long as you have the proof requirements, you'll win the dispute.
You'll find this useful: http://www.playerauctions.com/help/content/secure-seller-delivery-agreement/#788
Runescape currency specifically: http://www.playerauctions.com/help/content/delivery-evidence-requirements-currency-other-games/
So having two pictures with the trade screen of the gold and then accepted trade as the second screen is enough?

